1. Nature of Infectious Diseases

2. Understanding Infectious Diseases
Disease: the term refers to conditions that impair normal tissue function.
Cystic fibrosis
Atherosclerosis
Measles
All are diseases but there are fundamentally different causes for each disease
Epidemiology – study of disease in population

3. Infectious disease: a disease caused by the invasion of a host by agents whose activities harm the host’s tissues and can be transmitted to others (they are infectious)
Pathogens: disease producing agent
Normal flora: Bacteria in our body (We have more bacteria living in our bodies than we have cells!) They can be good for you!

4. The terms “infections” and “disease” are not synonymous.
An infection results when a pathogen invades and begin growing in the host
A disease results only if tissue function is impaired by the invasion and growth of a pathogen.
polio virus
Ebola hemorrhagic fever

5. Virulent: degree or intensity of pathogenicity (the ability to cause disease) of an organism. It is indicated by mortality rate from the related disease and/or ability to invade tissue and cause disease.
Salmonella typhi is a bacterial disease also called typhoid fever. It is contracted by eating or drinking contaminated food or water. People can be carriers of the disease for years after infection.

6. Infectious Disease Process
In order for a pathogen to cause disease they must be able to:
Enter the host body
Adhere to specific host cells
Invade and colonize host tissues
Inflict damage on host tissue

7. Entry to the host is usually through a natural orifice (mouth, eyes, genitals, wounds)
Attach to specific cells to invade other body tissues
Some pathogens inflict damage by growing; others produce destructive toxins

8. Modes of Transmission
May be transmitted through direct or indirect contact
Direct contact: occurs when an individual is infected by contact with the reservoir
Indirect contact: occcurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individual
Horizontal transmission: agent is passed from person to person
Vertical transmission: agent is transmitted from mother to baby

9. Disease reservoir: the environment where the infectious agent survives
Examples: humans or animals
Clostridium tetani is a bacteria disease and its reservoir is the soil
Yersenia Pestis is rodents and the vector was the flea

10. Microbes that cause infectious diseases
Bacteria: unicellular prokaryotic organism (no nucleus) that reproduce by growing and dividing into two cells – binary fission
a. bacillus (rod-shaped)
b. coccus (spherical)
c. spirillum (helical rods)
Virus: infectious agent composed of a protein coat, DNA or RNA. Lacks an independent metabolism and reproduces only within a host cell

11. Fungi: eukaryotic with rigid cellulose walls and reproduce primarily by forming spores
Protozoa: unicellular eukaryotes capable of a variety of rapid and flexible movements
Helminthes: simple, invertebrate animals that are infectious parasites. Drugs that kill helminthes are frequently toxic to humans.
Prions: infectious particles that consist only of proteins. Infections usually result in brain tissue that is riddled with holes. Creutzfeldt-Jakob disease is casued by prions.