Understanding infectious diseases

Understanding how infections and diseases spread can be very challenging. Here are some terms that can help. (Medecinenet.com)

Infection: The invasion and multiplication of microorganisms such as bacteria, viruses, and parasites that are not normally present within the body. An infection may cause no symptoms and be subclinical, or it may cause symptoms and be clinically apparent. An infection may remain localized, or it may spread through the blood or lymphatic vessels to become systemic (bodywide). Microorganisms that live naturally in the body are not considered infections. For example, bacteria that normally live within the mouth and intestine are not infections.

Pathogen: An agent of disease. For example, Bacillus anthracis is the pathogen that causes anthrax.

Host- 1. The organism from which a parasite obtains its nutrition and/or shelter.

         2. An organism or cell culture in which a virus can replicate itself.

Reservoir of infection: Any person, animal, plant, soil or substance in which an infectious agent normally lives and multiplies. The reservoir typically harbors the infectious agent without injury to itself and serves as a source from which other individuals can be infected. The infectious agent primarily depends on the reservoir for its survival. It is from the reservoir that the infectious substance is transmitted to a human or another susceptible host.

Zoonosis: An infection that is known in nature to infect both humans and lower vertebrate animals.

Many infectious agents have the ability to infect more than one host species, making it very difficult to track. 62% of all human pathogens are classified as zoonoses. The pathogens that infect livestock are 77% zoonotic. Fifty seven of the seventy animal diseases that are considered to be of greatest importance infect multiple hosts. These multi-host pathogens are hard to control and it is important to understand the term reservoir of infection. 

Often, emerging human or wildlife diseases are assumed to be maintained in a reservoir host; but are rarely defined. The control efforts are often directed toward “Suspected reservoir populations”.  Reservoirs that harbor Ebola are still unknown, making it difficult to eradicate the disease (Hayden, et. al, 2002).

If you are interested in this topic; you can read further by looking at this article:

Haydon, D., et. al., 2002. Identifying Reservoirs of Infection: A Conceptual and Practical Challenge. Emerging Infectious Disease. Vol 8(12). 1468-1473.

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