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