Background on Infectious Disease

• Attachment with or without entry into the body
• Logal or general spread within the body
• Multiplication
• Evasion of host defenses
• Shedding from the body (exit)
• Cause damage in the host

Microorganisms are evolving at a much faster rate compared to most host organisms.

They multiply quickly due to very short generation time.

They also have horizontal gene transfer including antibiotic resistance genes.

• Microbes with mechanisms for invading host body surfaces. - Viruses, certain bacteria.
• Microbes transferred via biting arthropods. - malaria, plague, typhus, yellow fever.
• Microbes transferred via skin wounds or animal bites. - Clostridia, rabies
• Microbes that are opportunistic and are only able to attack when the host's defenses are down.

• 1. The suspected pathogen should be present in all cases of the disease and absent from healthy animals.
• 2. The suspected organism must be grown in pure culture.
• 3. Cells from a pure culture are introduced in to a healthy animal.
• 4. The suspect organism must be reisolated and shown to be the same organism.

• Some microbes cannot be cultured in the lab - Treponema pallidum, viruses
• For some microbes there are no susceptible nonhuman hosts.