In biology, a vector is any organism that facilitates the spread of a disease. Vectors do not inherently cause diseases but are mere vehicles of the germs that cause disease. Vectors may include small insects or larger animals that carry viruses from one host to another.
An example of a vector in biology is the mosquito, which carries the virus that causes malaria. Another example of an insect vector is the tsetse fly, which carries trypanosomes from animals to human beings. Animals such as dogs and bats can sometimes be referred to as vectors since their bite may cause transmission of viruses that cause rabies.