Beware when traveling outside the country — the Centers for Disease Control and Prevention issued a dengue warning for the U.S. The agency said 41 New Jersey residents tested positive for dengue after traveling abroad.

The virus, which is spread by mosquitoes, has been surging worldwide, helped by climate change. In barely six months, countries in the Americas have already broken calendar-year records for dengue cases.

The World Health Organization declared an emergency in December, and Puerto Rico declared a public health emergency in March.

Dengue remains less common in the continental United States, but in the 50 states so far this year there have been three times more cases than at the same point last year. Most cases in the continental U.S. continue to be people who were infected while traveling internationally.

Dengue is caused by a virus spread by a type of warm weather mosquito that is expanding its geographic reach because of climate change, experts say.

The CDC says the most common symptom of the mosquito-transmitted viral disease is fever. It can also include muscle, bone and joint pain and nausea or vomiting. Symptoms of dengue usually begin within two weeks after being bitten by an infected mosquito, according to the CDC website.

Many infected people don’t get sick, but some experience headache, fever and flu-like symptoms. Severe cases can involve serious bleeding, shock and death. Repeated infections can be especially dangerous.

There is no widely available medicine for treating dengue infections.

When planning a vacation, the CDC recommends looking into country specific travel information.