Jefferson County health officials confirmed Monday three West Nile Virus cases in humans, the first to be reported, while Denver reported its first the same day.

“Thankfully, most people who get the virus don’t have symptoms, but some people can get really sick or even die from West Nile Virus,” James Rada, Jefferson County Environmental Health director, said in a statement. “With the recent death of a resident in a neighboring county and now human cases in Jefferson County, it’s even more important that Jeffco residents do what we can to avoid being bitten by mosquitoes.”

West Nile virus is spread by mosquito bites.

While most bitten will not develop symptoms, about one in five do. Symptoms typically appear three to 14 days after being bitten.

Symptoms include headache, fever, diarrhea, muscle weakness and other flu-like symptoms. In rare cases, the virus can cause severe illnesses, such as encephalitis or meningitis, and fatal in some.

Jefferson County health officials reported three human cases. Denver officials also reported their first human case Monday.

Last year, the state saw 76 cases in humans, state data shows. Of those, Denver County had just six cases. Larimer County five and JeffCo three.

The best way to reduce the risk of a mosquito bite is to apply insect repellant, wear long sleeves and pants and avoid outdoor activities at dusk and dawn.

Individuals 60 and older or those with certain health conditions are at greater risk of contracting the virus, according to state health officials.

Infected mosquitos have been found in half of the 16 counties tested this year, including Adams, Arapahoe, Boulder, Broomfield, Denver, Jefferson, Larimer and Weld counties.

A Lafayette resident died from the virus last month.

To date, 11 people have tested positive for West Nile virus, according to the Colorado Department of Public Health and Environment. But this data is reported only once a week and does not reflect the new cases in Jefferson and Denver counties.