West Nile virus season has officially begun on Long Island with the discovery of infected mosquitoes in Nesconset, Suffolk County health officials said Friday.

The mosquito sample was collected on July 1.

The mosquito-borne virus, which can cause mild to more serious illness in children and adults, was first detected in New York in 1999. In 2024, there were 21 human cases of West Nile virus reported in Suffolk.

Nine people have died in Suffolk from the virus since 2000. But others have mild symptoms and might not be aware they have the disease.

Additionally, the county’s mosquito surveillance program showed four samples positive for the more rare Jamestown Canyon virus, collected in June and on July 3, officials said.

The last time that virus was isolated from mosquito samples in Suffolk County was in 2023. In New York, there have been just five human cases reported between 2011 and 2024.

The symptoms of Jamestown Canyon virus are similar to those of West Nile virus. People who are bitten may feel fatigue, have a headache and a fever. In more serious cases, it can develop into encephalitis.

“The finding of Jamestown Canyon virus or West Nile virus in mosquito samples indicates the presence of that virus in the area,” Suffolk County Health Commissioner Dr. Gregson Pigott said in a statement. “While there is no cause for alarm, we advise residents to take precautions and cooperate with us in our efforts to reduce exposure to mosquito-borne diseases.”

Pigott advised people to protect themselves from mosquito bites by wearing long pants and long-sleeved shirts when entering an area with active mosquitoes, using repellent and emptying standing water that can serve as breeding sites for mosquitoes.