Summary

In this study session you have learned that:

• Communicable diseases are caused by infectious agents that can be transmitted to susceptible individuals from an infected person, or from other animals, objects or the environment.
• Infectious agents include helminths, protozoa, bacteria, viruses and fungi.
• Six factors are involved in the transmission of communicable diseases: the infectious agent, the reservoir, route of exit, mode of transmission, route of entry, and the susceptible host.
• A reservoir is a human, another animal, or a non-living thing (such as soil), where the infectious agent normally lives.
• Modes of transmission of an infectious agent can be directly through person-to-person contact, or across the placenta from mother to fetus. Indirect transmission can occur through air, vehicles such as water, food and contaminated objects, or via a vector such as a mosquito.
• A susceptible host is a person or animal who can develop infection if exposed to the infectious agent. Susceptibility is increased if exposure is high, or the host’s immunity is low.
• The natural history of an untreated communicable disease has four stages: stage of exposure, stage of infection, stage of infectious disease, and stage of outcome.