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

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: microorganisms that are capable of causing disease. A true pathogen is one that causes disease in virtually every susceptible host.

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 of an organism
Virulent: degree or intensity of pathogenicity of an organism. It is indicated by mortality rate from the related disease and/or ability to invade tissue and cause disease.

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 disease reservoir is the soil
Yersenia Pestis is rodents

10. Microbes that cause infectious diseases
Bacteria: unicellular prokaryotic organism (no organized internal membranous structure) that reproduce by growing and dividing into two cells
a. bacillus (rod-shaped)
b. coccus (spherical)
c. spirillum (helical rods)
Virus: infectious agent composed of a protein coat and a single type of nucleic acid. 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.