West Nile virus (WNV) is a mosquito-borne virus that can potentially cause serious illness. West Nile virus is considered to be permanent in Colorado and infections can be expected each year.


People of any age who spend a lot of time outdoors are more likely to be bitten by an infected mosquito and therefore develop West Nile disease. People over 50 years of age and those who have serious diseases and immune disorders are more likely than younger patients to have the most severe forms of the disease.


Signs of illness appear between from 3 - 14 days after a person is bitten by an infected mosquito.

About one in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

About 20 percent of the people who become infected have milder illness such as fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last anywhere from a few days to several weeks.

Approximately 80 percent of people who are infected with WNV will not show any symptoms at all.


WNV is transmitted to mosquitoes after they bite infected birds. You cannot catch the virus from another person.


Thousands of people in Colorado were infected with West Nile virus in 2003, the worst year in Larimer County. Sixty-three people died. Many people who became seriously ill are continuing to recover while others suffered permanent disabilities.

Infected mosquitoes have been trapped in Larimer County each year since 2003.

View the annual West Nile Virus history for Larimer County.