(NEW YORK) -- A record number of West Nile virus cases have been recorded for this time of year, the Centers for Disease Control and Prevention (CDC) warned on Wednesday ahead of the upcoming holiday weekend.

At least 48 cases of West Nile have been reported so far this year compared to an average of 10 typically by the end of June. This is the highest number of infections reported at this point in year since 2004, according to the federal health agency.

Additionally, at least 23 states are reporting West Nile activity, which is the highest number recorded over the last 10 years, CDC data shows.

The illness is the leading cause of mosquito-borne disease in the contagious U.S., with several thousand cases and over 100 deaths reported on average each year.

With millions gathering outside for Fourth of July, the CDC is urging Americans to take precautions and reduce their risk of mosquito bites.

Since 1999, the disease has killed more than 3,300 Americans. Cases have historically peaked in August and are mostly reported from June through October.

While many people infected do not develop symptoms, about one in five do. Symptoms include fever, headache, body aches, vomiting, diarrhea or rash, according to the CDC.  Most symptoms soon disappear, though weakness and fatigue may last for weeks or months.

Fewer than 1% of people infected develop severe illness that affects the central nervous system, including inflammation of the brain (encephalitis) and the membranes around the brain and spinal cord (meningitis), the CDC says.

Risk of severe disease, hospitalization and death increase with older age; certain medical conditions including cancer, high blood pressure and kidney disease; and a weakened immune system.

There are currently no vaccines for West Nile virus, nor disease-specific treatments. The CDC recommends rest, fluids and over-the-counter medications to treat the infection. Those with severe illness may need to be hospitalized and receive additional support treatments, such as intravenous fluids.

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