The mosquito-borne disease has shown up in 13 states so far. Experts offer advice on how to reduce your risk of contracting the ailment.

Health officials in the United States are reporting cases of West Nile virus as the season for the mosquito-borne infection is well under way.

West Nile virus activity usually begins in June in the United States, and has been occurring every summer since 1999.

Since the infection was first reported in North America it has been detected in all 50 states except Alaska and Hawaii.

According to data from the Centers for Disease Control and Prevention (CDC), there have been 32 reported cases of West Nile virus as of July 2 this year. Half of those were the more serious neuroinvasive form of the disease.

So far, human cases of West Nile virus have been reported in Indiana, Kansas, Mississippi, Tennessee, Texas, Arizona, Colorado, Nevada, New Mexico, South Dakota, Georgia, Alabama, and California.

Read more: What is West Nile virus infection? »

Most people who contract West Nile virus never develop serious symptoms. But experts say people should be aware of the risk of the infection.

“The numbers indicate that West Nile virus has become endemic in the U.S. now,” Dr. William Schaffner, professor of preventive medicine and infectious diseases at the Vanderbilt University School of Medicine, told Healthline.

“West Nile virus is serious in several ways. Some persons who become infected can develop quite serious illnesses, most ominously encephalitis, especially in older persons where it can be fatal or result in permanent disability on recovery. Even lesser illnesses, such as so-called ‘West Nile fever’ can be debilitating,” he added.

Most people become infected with West Nile virus after being bitten by an infected mosquito.

The mosquitoes become infected after feeding on infected birds. They then spread the virus to other animals and humans.

According to the Mayo Clinic, 20 percent of people who get infected develop mild symptoms, including fever, headaches, body aches and pains, diarrhea, and vomiting.

Less than 1 percent of infected people develop serious neurological infections like meningitis and encephalitis.

Some of the symptoms of the more serious cases include high fever, severe headache, stiff neck, confusion, and partial paralysis.

People over the age of 60 are at higher risk of severe disease, as are those with certain medical conditions like cancer, kidney disease, and diabetes.

There is no treatment or vaccine against West Nile virus.

The best method of protection from the virus is to wear insect repellent outdoors, as well as wear long sleeves and pants after dusk when mosquitoes are most active.

People are also urged to eliminate breeding sites for mosquitoes around your home. This involves emptying anything that may hold standing water like flowerpots or birdbaths.

Gonzalo Vazquez-Prokopec, PhD, an assistant professor in the department of environmental sciences at Emory University, said the virus is difficult to forecast and hard to contain.

“It is very hard to predict when and where transmission will occur,” he told Healthline.

Disparity in mosquito surveillance resources in different areas of the country is another factor that makes it difficult to contain the virus quickly.

“Areas with entomologic surveillance can use their data to respond with vector control when elevated signals of West Nile virus infection in mosquitoes is detected. Areas without such surveillance can only respond once cases are confirmed, generally weeks after transmission occurred,” he said.

The weather, the number of birds with the virus, the number of mosquitoes spreading the virus, and human behavior are all factors that influence where and when an outbreak takes place.

Even economic events have been shown to influence the spread of the virus.

“It has been shown that during the recent recession, many people could not pay their mortgage costs so they had to move out of their homes,” Schaffner said. “The housing market was slow and the homes did not sell and were not maintained. Many of these homes in the South and Southwest had swimming pools, which also were not maintained, [and] the water became ideal mosquito breeding sites.”

Uriel Kitron, PhD, works with Vazquez-Prokopec studying the transmission risk of arboviruses (viruses carried by mosquitoes).

Read more: Climate change could be devastating to global human health »

He believes the United States has been ill prepared for past outbreaks of West Nile virus, and isn’t prepared for outbreaks of what he says could come next, like Zika, Rift Valley fever, or an unknown emerging virus.

“West Nile virus should serve as a warning about the need to be prepared for the next big one … and we have not learned our lessons,” Kitron told Healthline.

He said there needs to be more investment in surveillance for the infection and more targeted protection against mosquitoes.

Schaffner holds a similar view.

“My major concern relates to the degree that local communities are capable of doing mosquito monitoring, surveillance of disease, and mosquito abatement. This is a broad concern beyond West Nile virus and includes Zika surveillance and control,” he said. “For many years, mosquito abatement was done just for summer nuisance mosquitoes, but the resources and expertise for serious disease control are substantially more demanding.”