West Nile virus, a dangerous mosquito-borne illness, has killed an adult from Lafayette, according to health experts from the Colorado Department of Public Health and Environment and Boulder County Public Health.

So far this summer, Colorado health officials have confirmed nine cases of West Nile virus in eight counties. And they have detected infected mosquitoes in 16 Colorado counties, including heavily populated areas in Adams, Arapahoe, Boulder, Broomfield, Denver, Jefferson, Larimer and Weld counties. (Learn more about West Nile virus cases in Colorado.)

“We are deeply saddened to confirm the first West Nile virus-related death in our state this year. This tragic loss underscores the serious nature of this virus and why personal prevention is so vital,” said Dr. Ned Calonge, chief medical officer at the Colorado Department of Public Health and Environment. “I cannot stress enough the importance of taking simple, effective steps — like using repellent and draining standing water — to protect yourself and your loved ones from West Nile virus infections.”

Last summer, Colorado health officials recorded 76 cases of West Nile virus, putting the state among the top five worst in the U.S. for West Nile cases. Texas recorded the highest number of West Nile cases in 2024, followed by California, New York and Nebraska, according to data from the U.S. Centers for Disease Control and Prevention.

In 2023, Colorado experienced the worst West Nile virus outbreak in the U.S. and one of the deadliest years for West Nile since the mosquito-borne illness arrived in the U.S. back in 1999.

Throughout 2023, 50 people who contracted West Nile virus died in Colorado, and scores of others needed to be hospitalized.

West Nile virus can strike healthy people of all ages. And it can cause scary neurological impairments and swelling in the brain in those who get a severe case. (Read about an award-winning photographer and producer who contracted West Nile virus in 2024 and a young woman who endured a tough recovery after a bad West Nile infection in 2022.)

Dr. Michelle Barron, senior medical director of infection prevention and control for UCHealth, has two key messages for people who are spending time outdoors now and for those who may be feeling sick.

Protect yourself to avoid getting mosquito bites. And, if you are feeling sick and suspect you might have West Nile virus, seek help from a doctor immediately.

“If your infection progresses to the point where you are having persistent headaches, confusion, any kind of muscle weakness, numbness or tingling, you need to be seen by a doctor as quickly as possible,” Barron said.

To understand more about West Nile virus and to prevent people from getting sick, we asked Barron some of your top questions about West Nile virus.

Along with her work for UCHealth, Barron is a professor of medicine and infectious diseases at the University of Colorado School of Medicine on the Anschutz Medical Campus and is one of the top infectious disease experts in the Rocky Mountain region.

What is West Nile virus?

West Nile virus is a mosquito-borne illness that’s known as a “flavivirus.” It was first detected in 1937 in Uganda. There were several large outbreaks in Egypt and the Nile River Delta in the 1950s, which is how the illness got its name. The first case in the U.S. was detected in 1999. It has since spread throughout the U.S. But Coloradans get a disproportionately high number of West Nile virus infections compared to people in other larger, more populated states. So, people living in Colorado and other states where West Nile virus infections are more common need to be especially vigilant about preventing mosquito bites.

Mosquitos can pick up the virus from birds and other animals and then spread it to people when they bite us.

Why was West Nile virus so bad in 2023 in Colorado?

Colorado had a wet spring and summer in 2023.

“The increase in rain has helped the mosquito populations to increase and spread more than usual this summer,” Barron said.

In addition, Coloradans love spending time outdoors. The combination of many people spending time in nature while mosquitos are spreading widely may contribute to the spread of West Nile virus.

What is the typical season for West Nile virus cases in Colorado?

The season usually lasts from June through September, but if we have a mild, wet fall, mosquitos may continue to circulate later in the fall, and thus, they could continue to infect people.

Is West Nile virus also bad elsewhere in the U.S.? 

It’s too soon to know how dangerous West Nile virus will be both in Colorado and across the U.S. this year. As of mid-July, CDC health officials had recorded about 50 cases in 15 states. But it takes time for state health officials to record cases of West Nile virus, and many people who get sick with West Nile virus don’t know they have it and thus, never report their illness to health officials.

Was 2023 one of the worst years on record for West Nile virus in Colorado?

Colorado has had bad years for West Nile virus in recent years. In 2023, 50 people who contracted West Nile virus died in Colorado. In 2022, there were 20 deaths, and in 2021, there were 11 deaths, according to the Colorado Department of Public Health and Environment.

The worst year on record in Colorado was soon after the West Nile virus arrived in the state. In 2003, 66 people died after becoming infected with the virus.

What are the common symptoms of West Nile virus?

The key symptoms of West Nile virus include:

• Dolor de cabeza
• Fiebre
• Diarrea
• Muscle weakness
• Tingling or numbness
• Neck stiffness
• Flu-like symptoms

“Most people will have minor symptoms and may not ever know that they had West Nile virus,” Barron said.

Do some people get rashes when they get West Nile virus?

Rashes are not common with West Nile virus, but it’s possible to get one. If you are not feeling well and have questions about symptoms, reach out to your doctor.

People with severe cases of West Nile virus can develop neurological symptoms. How would you know that you are having these kinds of issues?

The neurologic symptoms related to West Nile virus can include the following:

• Clumsiness
• Problems walking
• Brain fog or cognitive problems
• Memory issues

When a case of West Nile virus becomes severe, Barron said patients can suffer from critical problems like encephalitis, which is inflammation of the brain, and meningitis, which is swelling in the lining of the brain and the spinal cord.

“People sometimes will start acting strange. They’ll have cognitive issues. They might have trouble with word finding. They might be a little slower or will have some weaknesses develop. Walking can be a challenge,” Barron said.

“They can have a polio-like syndrome where they can’t walk and have paralysis,” Barron said.

Her advice for anyone having these types of neurologic symptoms is utterly clear: Seek help immediately.

Is there a test for West Nile virus?

Yes. If you are sick and suspect that you might have West Nile virus, see your medical provider so they can arrange for you to be tested for West Nile virus.

Are there any cures for West Nile virus?

No. Despite decades of research and efforts to come up with a cure, there’s no cure yet for West Nile virus. In the past, Barron said, doctors experimented with using antibodies from previously infected people in patients with new infections. Unfortunately, clinical trials showed that those therapies were not effective.

While there are no cures for West Nile virus, doctors can help with other symptoms, like reducing a patient’s fever or providing supportive care if they have swelling in the brain.

“If you’re having neurological symptoms, you want to make sure you see a doctor because the symptoms can get worse,” Barron said.

There’s no way to reverse the virus, but doctors can provide help.

“If you’re having trouble breathing, we can monitor that. If you’re having fevers, we can control that. If you’re having weakness, we can work on rehabilitation,” she said.

Is there a vaccine to prevent West Nile virus?

No. As with cures, researchers have been working for years to create a vaccine for humans to prevent West Nile virus. Unfortunately, there is not a vaccine yet. But there is a vaccine to prevent West Nile virus in horses, so it’s possible that vaccines for humans could come soon.

What if I had West Nile virus in the past? Could I get it again, or would I still have immunities to fight another infection?

Researchers and doctors are still learning about West Nile virus, but they think that once a person has West Nile virus, they should have lifelong immunity to the virus.

What can I do to avoid getting West Nile virus?

West Nile virus is highly preventable, but if you like being outside, it’s not easy to avoid mosquitos.

Here are some ideas for staying safe:

• If you’re spending time outdoors, consider wearing long sleeves and long pants, so it’s not easy for mosquitos to bite you.
• Use a highly effective insect repellent, preferably one containing DEET.

“If you’re out hiking or even if you’re out sitting on your deck or your patio, it’s a good idea to protect yourself. You can use insect repellent, citronella candles or anything that helps keep the mosquitos down,” Barron said. “And make sure there’s no standing water where mosquitos can breed.”

Are there certain times of day when you should be more cautious?

Yes. Mosquitos are especially active at dawn and dusk.

“Avoiding being outside at those times may help you avoid West Nile virus,” Barron said.

Overall, her advice is to prevent bites. Be prepared and pay attention.