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Health officials are ramping up warnings to the public, because most cases of the mosquito-borne disease are reported in August and September.

“West Nile virus can be a very serious disease and its presence in mosquitoes remains high right now in Massachusetts,� said the state’s public health commissioner, Dr. Robbie Goldstein, in a statement last week.

People can protect themselves by wearing long-sleeved shirts and pants when possible, and using an EPA-registered when spending time outdoors, health officials say.

West Nile virus was first reported in the United States in 1999 in New York, and then gradually spread across the country. It in 2003, when nearly 10,000 cases were reported.

Scientists say many people — perhaps tens of thousands each year — are infected but don’t know it because they have no symptoms, or only mild ones such as headaches, body aches, joint pain, vomiting, diarrhea and rashes.

In severe cases, damage to the central nervous system causes inflammation of the brain or spinal cord, and even death.

In the last decade, health officials have fielded reports of 2,000 cases annually on average, including 1,200 life-threatening neurological illnesses and approximately 120 deaths. Deaths are on pace to be higher this year, CDC officials said, but they declined to discuss specifics, saying mortality statistics are too preliminary.

The problem is not that there are more mosquitoes this year, but rather that a higher proportion of the bugs are carrying the virus, CDC officials said. Mosquito infection rates can be affected by such factors as temperature, rainfall, the amount of insect control going on, and how many nearby birds are infected.

Colorado, which tends to see more West Nile virus, has reported about 150 of the nation’s cases — more than double what other states are reporting.

Fort Collins is a hot spot. Monitoring last month in an area in the southwest part of the city found that 35 out of every 1,000 female mosquitoes were infected — far higher than the 8 per 1,000 that would be expected for that time of year — said Roxanne Connelly, a CDC entomologist who lives there.

It's not clear why, but she noted it's been a wet and warm year.

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