Washington ? U.S. health officials are warning that a sometimes-deadly tropical disease that’s spread by mosquitoes is re-emerging worldwide and could gain a foothold in the U.S. one day.

Dengue, a flu-like illness that infects 50 million to 100 million people a year, has been growing more prevalent and severe as it moves from tropical regions into more temperate areas such as Puerto Rico, where it’s now endemic, and along the U.S. border with Mexico.

An estimated 21 people are thought to have died from dengue fever last year in Puerto Rico, where the number of cases jumped to more than 10,000 in 2007 from about 3,000 the year before, according to the Centers for Disease Control and Prevention. More than 30,000 cases were reported in Mexico last year.

Despite these and other sporadic outbreaks, dengue (pronounced DENG-ee) hasn’t established itself in the continental United States. But a number of factors suggest that one day it could.

Expanded migration of a mosquito that transmits the disease, increased urbanization, and rising temperatures and rainfall – possibly caused by global warming – have helped fuel an alarming global resurgence of the disease. This increases the likelihood that it could strike even harder in the U.S.

The CDC estimates that 100 to 200 cases each year are introduced into the United States by travelers.

“The U.S. is not immune to vector-borne viruses” – those spread by insects or animals – “and dengue re-emerging globally should be an eye-opener that it could be the next West Nile virus that hits the United States. It’s endemic in Mexico. It’s endemic in Puerto Rico. It’s all throughout the Caribbean. It’s knocking on our door,” said Barry W. Alto, a postdoctoral associate in the department of ecology and evolutionary biology at Yale University.

People infected with dengue can have no symptoms or mild to high fevers, severe frontal headaches, severe joint pain and pain behind the eyes. Nausea, vomiting and rashes also can occur.

A more severe form of the disease, dengue hemorrhagic fever, features similar symptoms along with bleeding from the skin, nose or gums and possible internal bleeding. In some cases of hemorrhagic fever, the capillaries can become leaky, allowing fluid to drain from the blood vessels. If untreated, this can lead to circulatory-system failure, followed by shock and sometimes death. With proper treatment, death rates from severe dengue are less than 1 percent.

There’s no vaccine for dengue, but doctors often prescribe nonaspirin pain relievers along with rest and increased fluids. Researchers at the University of Pittsburgh Medical Center are working on a dengue-fever vaccine with a $4.8 million grant from the U.S. Department of Defense.

Reported dengue cases have been increasing worldwide since the mid-1950s, but most are found in the tropical countries of Asia, the South Pacific, Africa, the Caribbean and the Americas.

The World Health Organization estimates that roughly 500,000 people are hospitalized and 22,000 die each year from dengue and related complications.

Dengue isn’t spread from person to person, but by mosquitoes that get the disease by biting infected people. The so-called yellow fever mosquito, found mainly in tropical regions, is the primary transmitter of dengue along with the Asian tiger mosquito, which is now found in 36 states after first appearing in the U.S. in 1985.