Infectious Diseases

• Pathogens: Microorganisms that are capable of causing disease

• Infection: Results when a pathogen invades and begins growing within the host

• Disease: Results only if and when normal tissue function is impaired

• The body has defense mechanisms to prevent infection (i.e. burns, skin lesions)

• In order to cause disease, pathogens must be able to enter, adhere, invade, colonize, and inflict damage

• Entrance to the host typically occurs through natural orifices such as the mouth, eyes, genital openings, or through wounds that breach the skin barrier to pathogens

• Growth of pathogens or the production of toxins/enzymes cause disease

• Some normal flora prevent diseases

Infectious Disease – a disease caused by the invasion of a host by pathogens causing impaired tissue function and can be transmitted to other individuals

Five major types of infectious agents (microbes): bacteria, viruses, fungi, protozoa, and helminthes:

• Bacteria: They contain no organized internal membranous structures. Most reproduce by growing and dividing into two cells in a process known as binary fission.

Types of Bacteria:

• Salmonella typhi: a gram-negative organism that causes typhoid fever.

• Yersinia pestis- Causes plague The reservoir is rodents. ***

• Staphylococcus aureus- causes skin, respiratory and wound infections.

• Viruses: Infect all organisms from plants and animals to fungi and bacteria. They are not organisms themselves because apart from a host cell, they have no metabolism and cannot reproduce.***

Types of viruses:

• Herpes viruses: cause chicken pox, cold sores, and painful genital lesions, and the pox virus that causes smallpox.

• Rhinoviruses: cause most common colds.

Viruses (continued)

• Myxoviruses and paramyxoviruses: cause influenza, measles, and mumps.

• Rotaviruses: cause gastroenteritis.

• Retroviruses: cause AIDS and several types of cancer.

Fungi: Reproduce primarily by forming spores.

Types of diseases caused by fungi:

• Ringworm

• Histoplasmosis (a mild to severe lung infection transmitted by bat or bird droppings)

• Candida genus: opportunistic pathogens*** that may cause diseases such as vaginal yeast infections and thrush.

Protozoa: Do not have cell walls and are capable of a variety of rapid and flexible movements.

• Can be acquired by contaminated food or water or by the bite of an infected arthropod such as a mosquito.

Helminths: Simple, invertebrate animals, some of which are infectious parasites. Difficult to treat because the drugs that kill helminthes are frequently very toxic to human cells.

Diseases caused by helminths:

• Trichinella Spiralis: occurs when improperly cooked pork from infected pigs is ingested.

– Symptoms include vomiting and diarrhea and fever***

– Respiratory paralysis can occur in fatal cases of trichinella spiralis***

Prions: Infectious particles that consist of only protein.

•Diseases caused by Prions:

• Creutzfeldt-Jakob disease (in humans)***

•Bovine spongiform encephalopathy (mad cow disease)

• Epidemiology- the study of the occurrence of disease in populations.

•Disease reservoirs- The reservoir for a disease where the infectious agent survives (example: rodents, soil

•Modes of transmission: Infectious agents may be transmitted through either direct or indirect contact.

Host defenses against infectious diseases:

• Nonspecific mechanisms-the body’s primary defense against disease – these include anatomical barriers to invading pathogens, physiological deterrents to pathogens, and presence of normal flora. (skin, low pH and high salinity)

• Specific mechanisms of host resistance- our immune system

• Immunity – when a host encounters an antigen that triggers a specific immune response for the second time and the body responds quickly and produces antibodies

• Vaccination- produces immunity