Indirect Transmission

Diseases that are transmitted to humans indirectly via an insect, an arthropod (animals with jointed appendages and exoskeletons, such as ticks), or another animal (such as snails, which deliver the parasite responsible for schistosomiasis) are called vector-borne diseases. Vectors carry disease-causing viruses, bacteria, or parasites from one host to another, delivering these pathogens to humans and other warmblooded hosts. The vectors themselves typically suffer no ill effects from the organisms they carry.

American dog tick, potential vector of Rocky Mountain spotted fever.

American dog tick, potential vector of Rocky Mountain spotted fever.

In 1999, for example, a mosquito-borne infection—West Nile virus—suddenly began targeting New Yorkers. Seven people died and 62 were hospitalized. Until then the virus had been confined to Africa, West Asia, and the Middle East. Today, the infection caused by West Nile virus has fully established itself in North America, flaring up in the summer and continuing into the fall. Since 1999 the virus has spread rapidly across North America and into Latin America. In 2009 there were 720 reported cases of West Nile virus in the United States, of which 32 were fatal.


Wild or domestic animals are natural reservoirs for many vector-borne diseases. The main reservoir host for West Nile virus is wild birds. The New York City strain of the virus was virtually identical to a strain taken the previous year from a dead goose in Israel. Scientists speculate that an infected mosquito, human, or bird may have brought the pathogen to this country on a plane or ship.

A vector ecologist tests crows for the West Nile virus in a lab in St. Paul, Minnesota. Dead crows were one of the early signs of the virus’s presence in North America.

A vector ecologist tests crows for the West Nile virus in a lab in St. Paul, Minnesota. Dead crows were one of the early signs of the virus’s presence in North America.

Many other common infections, including malaria, yellow fever, Lyme disease, and typhus, are spread to humans from animals via the bites of insects and other arthropods. In fact, nearly half the world’s population is currently infected with a vector-borne disease.



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