It starts with a seemingly innocuous mosquito bite — often not even noticed.
Four or five mornings later, the aching sets in. A reddish rash can spread over arms, legs, and face. Fever spikes. Some patients complain of headaches.
Mostly, though, it's the joint pain they later remember. Legs that throb like they've just run a marathon, arms that feel like they've competed at rugby.
For most people, a bout of the mosquito-borne disease chikungunya ends about a week after it starts. For a small percentage of people, the joint pain can linger for weeks, months, or even forever.
The virus — pronounced chick-un-GOON-ya — used to infect people mainly on the other side of the world. But over the last year, it has rampaged through the Caribbean and Latin America infecting nearly 1 million people. It has spread into Florida and Puerto Rico, and threatens to keep moving northward.
"Whatever comes to the Caribbean and Latin America generally makes its way to the mainland United States," said Dr. Mark Gendreau, vice chairman of emergency medicine at Lahey Hospital and Medical Center in Burlington.
The infection is rarely lethal, but the pain can be debilitating.
Although only a handful of cases have been reported in Massachusetts over the last six months — one or two each at Tufts, Massachusetts General Hospital, and Boston Medical Center — infectious disease doctors are anticipating more this winter as people head for warmer climates for vacation.
"It's just a matter of time," Gendreau said.
As of late last month, according to the Pan American Health Organization, more than 120,000 people had been infected in El Salvador, nearly half a million in the Dominican Republic, and thousands in the vacation destinations of Jamaica, Barbados, and Curacao.
Doctors say it's relatively easy for travelers to protect themselves if they're willing to cover up and wear mosquito repellent.
Luckily for beach-goers, the mosquitoes don't like bright sunlight, and only hover around sources of fresh water. So lying on a beach should be safe, unless there are, say, large flower pots that have been generously watered or other pools of standing water nearby. Gendreau suggests golfers wear long sleeves, pants, and bug spray.
The mosquitoes that carry chikungunya — Aedes aegypti and Aedes albopictus — are the same species that can carry dengue, West Nile, and Eastern Equine Encephalitis. Unlike mosquitoes that carry malaria, the ones that bring chikungunya are active during the day.
There are no pills that can protect against chikungunya. And there is no specific treatment for the virus, other than supportive care, such as pain relievers and extra fluids.
But the absence of preventive drugs or treatment doesn't mean people should skip the doctor if they think they might be infected, said Dr. Natasha Hochberg, co-director of the travel clinic at Boston Medical Center. It's important to rule out other potentially life-threatening diseases, she said.
Hochberg said she sees no reason for people traveling to Florida to worry — their chances of contracting chikungunya are small — but people headed farther south should take precautions, such as covering their limbs and wearing bug spray.
Although the United States has been largely immune to tropical diseases in recent decades, the changing climate, more extensive travel, and the mosquito's ability to adapt may not protect us much longer.
"Chikungunya is a classic example of the potential for extensive spread of infectious diseases," said Dr. Edward T. Ryan, director of Tropical Medicine at Massachusetts General Hospital. "It just reinforces that there are constantly emerging infectious diseases."
About 100 people have already brought chikungunya to Florida, where they then were bitten by local mosquitoes, allowing the virus to spread there, Lahey's Gendreau said. In Massachusetts, where the freezing winters kill off most mosquitoes, we probably won't see widespread chikungunya infections, he said.
But as mosquitoes adapt "and become more comfortable in moderate to even cold climates, those diseases we normally would just see in tropical areas become more prominent here," Gendreau said.
Chikungunya was first described in 1952 in southern Tanzania, where it was named in the local language, meaning "to become contorted" — which described what people looked like when they were bent over in pain caused by the illness.
The virus spread across the Indian Ocean in recent decades, infecting millions in India, and arrived in the Caribbean last December. In mosquito-prone areas, chikungunya usually rips through a population, infecting large percentages, said Mass. General's Ryan. Those who get chikungunya generally become immune after their first infection.
So far, drug development has been slow, in part because the disease is rarely lethal, and has been concentrated in countries facing more serious diseases, like malaria.
But in 2007, the virus struck a small French-owned island in the Indian Ocean called La Réunion, said Dr. Erich Tauber, a European vaccine developer.
Within 14 months of its arrival on the island, about 40 percent of the 800,000 residents had caught the disease, and European companies started to think seriously about developing a vaccine, said Tauber, CEO of Themis Bioscience, an Austrian biotech that recently began testing a possible vaccine in people.
Themis's vaccine test went well, research shows, with only minor side effects reported. If testing in more people proves safe and effective, the vaccine could be available to the public in about four years, Tauber said.
In the meantime, Nita Bhat suggests that others take the precautions she didn't. In 2009, Bhat went to her birth village in India and after neglecting to use bug spray or wear long sleeves got bitten by a mosquito carrying chikungunya.
Then a student at Stanford University, Bhat woke up her last morning in India feeling sore all over. By the time she had flown the 2½ hours from Bangalore to Delhi, she couldn't walk and needed a wheelchair to reach her connecting flight.
Bhat, now an educational consultant in Boston, was one of the unlucky ones whose disease lingered. For most of that year at Stanford, she needed a golf cart to get around campus and had lingering pains and terrible fatigue. Slowly, over the next six months, the aches subsided and her energy returned.
"I don't want anyone else to get chikungunya," Bhat said recently. "I think it's a miserable disease."
Karen Weintraub can be reached at email@example.com.