Vectors are frequently arthropods, such as mosquitoes, ticks, flies, fleas and lice.

Vectors can transmit infectious diseases either actively or passively:

• Biological vectors, such as mosquitoes and ticks may carry pathogens that can multiply within their bodies and be delivered to new hosts, usually by biting.
• Mechanical vectors, such as flies can pick up infectious agents on the outside of their bodies and transmit them through physical contact.

Diseases transmitted by vectors are called vector-borne diseases

Many vector-borne diseases are zoonotic diseases, i.e. diseases that can be transmitted directly or indirectly between animals and humans. These include for example tick-borne Encephalitis and Leishmaniosis. Many vector-borne diseases are considered as emerging infectious diseases:

• Disease that appears in a population for the first time.
• That may have existed previously but is rapidly increasing in incidence or geographic range.

Some vectors are able to move considerable distances. This may affect the transmission ranges of vector-borne zoonotic diseases. Vectors can be introduced to new geographic areas for example by:

Travel of humans and international trade;

• Animal movement, for instance of livestock;
• Migratory birds;
• Changing agricultural practices;
• By the wind movement.