HSC Biology Module 7: Infectious Disease

Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful. But under certain conditions, some organisms may cause disease.

Some infectious diseases can be passed from person to person. Some are transmitted by insects or other animals. And you may get others by consuming contaminated food or water or being exposed to organisms in the environment.

Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue. Mild infections may respond to rest and home remedies, while some life-threatening infections may need hospitalization.

Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from most infectious diseases.

Infectious diseases can be caused by:

Bacteria. These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.

Viruses. Even smaller than bacteria, viruses cause a multitude of diseases ranging from the common cold to AIDS.

Fungi. Many skin diseases, such as ringworm and athlete's foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.

Parasites. Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal faeces.

These germs can also spread through the exchange of body fluids from sexual contact. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

______________________________________________________________________________________________

The Differences Between Bacteria and Viruses

Bacteria are relatively complex, single-celled creatures, many with a rigid wall, and a thin, rubbery membrane surrounding the fluid inside the cell. They can reproduce on their own. Fossilized records show that bacteria have existed for about 3.5 billion years, and bacteria can survive in different environments, including extreme heat and cold, radioactive waste, and the human body.

Most bacteria are harmless, and some actually help by digesting food, destroying disease-causing microbes, fighting cancer cells, and providing essential nutrients. Fewer than 1% of bacteria cause diseases in people.

Viruses are tinier: the largest of them are smaller than the smallest bacteria. All they have is a protein coat and a core of genetic material, either RNA or DNA. Unlike bacteria, viruses can't survive without a host. They can only reproduce by attaching themselves to cells. In most cases, they reprogram the cells to make new viruses until the cells burst and die. In other cases, they turn normal cells into malignant or cancerous cells.

Also unlike bacteria, most viruses do cause disease, and they're quite specific about the cells they attack. For example, certain viruses attack cells in the liver, respiratory system, or blood. In some cases, viruses target bacteria.

The discovery of antibiotics for bacterial infections is considered one of the most important breakthroughs in medical history. Unfortunately, bacteria are very adaptable, and the overuse of antibiotics has made many of them resistant to antibiotics.

Antibiotics are not effective against viruses, and many leading organizations now recommend against using antibiotics unless there is clear evidence of a bacterial infection.

Since the beginning of the 20th century, vaccines have been developed. Vaccines have drastically reduced the number of new cases of viral diseases such as polio, measles, and chickenpox.

 ______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

Pasteur's experiment 1862First, Pasteur prepared a nutrient broth similar to the broth one would use in soup.Next, he placed equal amounts of the broth into two long-necked flasks. He left one flask with a straight neck. The other he bent to form an "S" shape.

Then he boiled the broth in each flask to kill any living matter in the liquid. The sterile broths were then left to sit, at room temperature and exposed to the air, in their open-mouthed flasks.

After several weeks, Pasteur observed that the broth in the straight-neck flask was discoloured and cloudy, while the broth in the curved-neck flask had not changed.

He concluded that germs in the air were able to fall unobstructed down the straight-necked flask and contaminate the broth. The other flask, however, trapped germs in its curved neck, preventing them from reaching the broth, which never changed colour or became cloudy.

If spontaneous generation had been a real phenomenon, Pasteur argued, the broth in the curved-neck flask would have eventually become reinfected because the germs would have spontaneously generated. But the curved-neck flask never became infected, indicating that the germs could only come from other germs.

Pasteur's experiment has all of the hallmarks of modern scientific inquiry. It begins with a hypothesis and it tests that hypothesis using a carefully controlled experiment. This same process -- based on the same logical sequence of steps -- has been employed by scientists for nearly 150 years. Over time, these steps have evolved into an idealized methodology that we now know as the scientific method. After several weeks, Pasteur observed that

the broth in the straight-neck flask was discoloured and cloudy, while the broth in the curved-neck flask had not changed.

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

______________________________________________________________________________________________

__________________________________________________________________________

Transmission of Pathogens

Pathogens can be transmitted either directly or indirectly. Direct transmission involves the spread of pathogens by direct body to body contact. Direct transmission can occur from mother to child as exemplified with HIV, Zika, and Syphilis. This type of direct transmission (mother-to-child) is also known as vertical transmission. Other types of direct contact through which pathogens can be spread include touching (MRSA), kissing (herpes simplex virus), and sexual contact (human papillomavirus or HPV). Pathogens can also be spread by indirect transmission, which involves contact with a surface or substance that is contaminated with pathogens. It also includes contact and transmission through an animal or an insect vector. Types of indirect transmission include:

Airborne - pathogen is expelled (typically by sneezing, coughing, laughing, etc.), remains suspended in air, and is inhaled by or comes in contact with respiratory membranes of another person.

Droplets - pathogens contained in droplets of body fluid (saliva, blood, etc.) contact another person or contaminate a surface. Saliva droplets are most commonly spread through sneezing or coughing.

Foodborne - transmission occurs through eating contaminated food or by improper cleaning habits after handling contaminated food.

Waterborne - pathogen is spread by consumption or contact with contaminated water. Zootonic - pathogen is spread from animals to humans. This includes insect vectors

that transmit disease through biting or feeding and transmission from wild animals or pets to humans.

Occasionally, the amount of disease in a community rises above the expected level. Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

Koch's postulates: In 1890 the German physician and bacteriologist Robert Koch set out his criteria for judging whether a given bacteria is the cause of a given disease. Koch's criteria brought some much-needed scientific clarity to what was then a very confused field.

Koch's postulates are as follows:

The bacteria must be present in every case of the disease. The bacteria must be isolated from the host with the disease and grown in pure

culture. The specific disease must be reproduced when a pure culture of the bacteria is

inoculated into a healthy susceptible host. The bacteria must be recoverable from the experimentally infected host.

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

Small Pox – The Eradication of Variola VirusAn estimated 300 million people died from smallpox in the 20th century alone.

The earliest physical evidence of smallpox is the pustular rash on the mummified body of Pharaoh Ramesses V of Egypt, who died in 1157 BC. Traders carried the disease from Egypt to India. From there it swept into China in the 1st century AD and reached Japan in the 6th century. Returning crusaders provided a way for smallpox to spread through Europe in the 11th and 12th centuries. In the 18th century smallpox decimated the aborigines when it reached Australia, the last corner of the world to have escaped its ravages.

People struggled to find ways to battle with smallpox. Variolation was a process developed in the 10th century in China and India. It involves taking pus from the pocks of someone suffering from smallpox and inoculating healthy people with it. Usually a mild case of smallpox developed, giving lifelong immunity afterwards. There was a risk of death from this, but in a world where smallpox was rife the odds made it worthwhile; about 0.5-2 percent of people died after variolation, compared with 20-30 per cent after natural smallpox. A major disadvantage of the practice was that variolated people could pass on severe smallpox to others.

The real breakthrough in fighting the virus came in 1796, when Edward Jenner carried out his famous experiment. He inserted pus extracted from a cowpox pustule on the hand of a milkmaid, into an incision on the arm of an eight-year-old boy, James Phipps. Jenner was testing his theory, drawn from the folklore of the countryside, that milkmaids who suffered the mild disease of cowpox never contracted smallpox. Jenner proved conclusively that contracting cowpox provided immunity against smallpox as well. He was quick to realise the enormous potential of vaccination. In 1801 he wrote 'That the annihilation of the Small Pox, the most dreadful scourge of the human species, must be the final result of this practice.’

The last natural case of smallpox was in Somalia in October 1977. Ali Maow Maalin, a 23-year-old hospital, had never been successfully vaccinated. After his diagnosis, an intensive tracing and vaccination campaign led to 54,777 people being vaccinated in the next two weeks. The disease was cornered, with no vulnerable hosts nearby to spread to. Smallpox was declared eradicated.

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________

_______________________________________________________________________________________________