Vaccine: A biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and it is often made from weakened or killed forms of the microbe. The agent stimulates the body’s immune system to recognize the agent as foreign, destroy it, and “remember” it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

Vector: A carrier—especially an arthropod—that transfers an infective agent from one host (which can include itself) to another.

Vector-borne: Transmitted from one host to another by a vector.

Viremia: The presence of virus in the blood of the host.

Virus: A small infectious agent that can only replicate inside the cells of another organism. Viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea.

Water-based diseases: Diseases (e.g., schistosomiasis, Guinea worm disease) transmitted by a vector that spends part of its life cycle in the water and another part as parasites of humans and animals.

Water-washed diseases: Those diseases whose transmission is facilitated by insufficient quantities of water (regardless of its quality) for personal and domestic hygiene.

Yaws: An infection caused by Treponema pertenue that is a significant public health problem in three countries of the Southeast Asia region.

Zoonoses: Microbes that are naturally transmitted between animals and humans that cause disease in human populations but can be perpetuated solely in nonhuman host animals (e.g., influenza, rabies).

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