The definition of a vector is any animal capable of producing discomfort or injury, including, but not limited to, mosquitoes, flies, other insects, ticks, mites, and rats but not including domestic animals according to the California State Health and Safety Code, Section 2002(K).

Vectors have the ability to transmit infectious organisms that cause human and animal disease. These diseases can be serious and possibly fatal. In Butte County several mosquito species are capable of transmitting West Nile virus (WNV), Saint Louis encephalitis (SLE), western equine encephalomyelitis (WEE), can transmit a variety of pathogens such as ehrlichiosis, babesiosis, Rocky Mountain spotted fever, and Lyme.