Learn about mosquito and tick biology and how these animals can spread diseases to humans
What is a disease vector?
Biology of ticks and how they spread disease

What Are Vector-Borne Diseases and Why Do They Matter?

What is a Vector?

The Merriam-Webster dictionary describes a vector as "an organism (such as an insect) that transmits a pathogen." Vectors can include insects, like mosquitoes and blackflies, as well as other arthropods, like ticks and mites.

Pathogens are disease-causing agents, like bacteria, viruses, and parasites.

What is a Vector-Borne Disease?

The World Health Organization defines vector-borne diseases as "human illnesses caused by parasites, viruses and bacteria that are transmitted by mosquitoes, sandflies, triatomine bugs, blackflies, ticks, tsetse flies, mites, snails and lice."  These diseases can include malaria, Lyme disease, and West Nile virus.

A host is an organism (such as a mouse, deer, or human) who carries a pathogen once infected.
Two important vectors that transmit diseases to humans in the United States are mosquitoes and ticks.

They can carry different pathogens that cause different illnesses. You can learn about these vectors and the diseases they carry by looking at our Ticks and Mosquitoes in the United States page.
Biology of mosquitoes and how they spread disease

Biology of Mosquitoes and How They Spread Disease

The Life Cycle of a Mosquito

Source: CDC, Division of Vectorborne Diseases
Mosquitoes have 4 life stages:
  1. Eggs - Mosquitoes lay their eggs in water or damp soil. They tend to lay about 100 to 300 eggs at a time. After being submerged in water, mosquito eggs hatch into larvae.
  2. Larva - Larva (plural is larvae) feed on microorganisms in the water. They molt 3 times and then become pupae.
  3. Pupa - Pupa (plural is pupae) continue to develop in water until an adult flying mosquito emerges and leaves the water.
  4. Adult - Male adult mosquitoes eat nectar from flowers. Female adult mosquitoes feed on blood from humans and animals; they need this blood to make eggs.

You can learn more about the life cycle of Aedes mosquitoes by reading this guide produced by the CDC: Mosquito Life Cycle

How Mosquitoes Spread Disease

Female mosquitoes spread disease in their adult stage through the process of feeding.

Female mosquitoes become infected when they feed off host animals that are already infected with a pathogen. The infected female mosquito can then spread the pathogen to the next animal it bites through its saliva.

The diagram below was developed by the CDC, and shows how a mosquito can transmit West Nile virus from infected birds to humans and other animals.

Biology of Ticks and How They Spread Disease

The Life Cycle of a Tick

Source: CDC, Division of Vectorborne Diseases
Most ticks have 4 life stages:
  1. Eggs - Ticks can lay up to several thousand eggs at a time. Female ticks lay eggs after they have a blood meal and detach from their host animal. When tick eggs hatch, larvae emerge.
  2. S ix-Legged Larva - Tick larvae are rarely infected with illness-causing pathogens, but they can pick up pathogens during their first blood meal. After feeding, tick larvae molt into nymphs.
  3. Eight-Legged Nymph - Ticks feed for the second time in their nymph stage. The nymph stage is the most dangerous for transmitting illness-causing pathogens to humans because they are very small and hard to see and remove. After feeding, tick nymphs molt into adults.
  4. Adult - Ticks feed for the third and last time in their adult stage. After feeding, male and female adult ticks mate. The male usually dies after mating, while the female lays her eggs.

It can take a tick up to 3 years to complete its full life cycle.

Ticks must eat blood at each of these four stages to survive.

You can learn more about tick life stages at the CDC website for the Life Cycle of Hard Ticks that Spread Disease

Blackkeged tick in questing position (Source: CDC)

How Ticks Find Animal Hosts

Ticks find their hosts by sensing breath, body odors, body heat, moisture, and/or vibrations of animals.

Ticks pick a place to wait, like grasses and shrubs, and sit in a position called 'questing'. Ticks can't fly or jump.

When an animal brushes the spot where a tick is waiting, the tick grabs and climbs onto the animal.

Ticks can feed on mammals, birds, reptiles, and amphibians.
Life Cycle of the Blacklegged Tick

How Ticks Spread Disease

Like mosquitoes, ticks transmit disease through the process of feeding.

Ticks become infected when they feed off host animals that are already infected with a pathogen. The infected tick can then spread the pathogen to the next animal it bites through its saliva.

Ticks present the greatest risk to humans in their nymph and adult stages.