Federal health officials have issued a travel alert about the spread of yellow fever in Brazil and say travelers need to be vaccinated before heading to affected areas.
People may have to plan ahead because there is a shortage of yellow fever vaccine, the Centers for Disease Control and Prevention noted in its alert.
The outbreak of yellow fever has killed at least 40 people and possibly more than 80 people, the World Health Organization says. Some cases couldn't be confirmed, but the mosquito-borne virus has infected more than 400 people, WHO says.
Brazilian health officials say it's also killing monkeys - more than 400 in the southeastern state of Espirito Santo, just north of Rio de Janeiro state.
It's the worst outbreak of the mosquito-borne virus in Brazil since 2000, WHO says. And it's affecting parts of Brazil that had not been at risk of yellow fever spread in decades, so even frequent travelers to Brazil need to be aware.
"Anyone 9 months or older who travels to these areas should be vaccinated against yellow fever," the CDC said in the new alert.
"People who have never been vaccinated against yellow fever should not travel to areas with ongoing outbreaks."
Most people don't need a booster shot if they've been vaccinated once, the CDC said.
"However, a booster dose may be given to travelers who received their last dose of yellow fever vaccine at least 10 years ago and who will be in a higher-risk setting, including areas with ongoing outbreaks," it said.
"Because of the ongoing outbreak, travelers to the Brazilian states of Minas Gerais, Espirito Santo, and parts of Bahia, Sao Paulo, and Rio de Janeiro states may consider getting a booster if their last yellow fever vaccination was more than 10 years ago. Because of a shortage of yellow fever vaccine, travelers may need to contact a yellow fever vaccine provider well in advance of travel.
The vaccine can itself cause side-effects, sometimes serious, so the CDC says older people, pregnant women and others with weakened immune systems need to check with a doctor before being vaccinated.
Yellow fever is carried by mosquitoes.
Yellow fever is a relative of the dengue and Zika viruses but is far deadlier. It's also the only virus in the family that has a good vaccine to prevent it.
It cannot be eradicated since it not only infects people, but monkeys and other animals too, so the mosquitoes that spread it can be re-infected even if the population's vaccinated. It's spread both by the urban Aedes species and by jungle-dwelling Haemogogus mosquitoes.
Symptoms of yellow fever infection include fever, chills, headache, backache, and muscle aches. About 15 percent of people who get yellow fever develop hemorrhagic fever symptoms that can include bleeding, shock, organ failure, and sometimes death.
Brazilian health officials are conducting mass vaccination campaigns now and working to eradicate Aedes aegypti mosquitoes in cities and towns.
WHO helped stop an outbreak of yellow fever in Angola and the Democratic Republic of the Congo last June and July with a big vaccination campaign. Because of the vaccine shortage, doses were diluted. A diluted vaccine can protect quickly but not for as long as a full dose of vaccine.
Travelers to areas affected by yellow fever, dengue or Zika are advised to protect themselves against bites by using mosquito repellant, preferably one containing DEET, picaridin, oil of lemon eucalyptus (OLE), IR3535, or 2-undecanone (methyl nonyl ketone).
Long sleeves and long trousers can also protect. "Use permethrin-treated clothing and gear (such as boots, pants, socks, and tents). You can buy pre-treated clothing and gear or treat them yourself," CDC advises.
The threat of yellow fever is likely to worsen, WHO says.
"Outbreaks like the one in Angola and the Democratic Republic of the Congo could become more frequent in many parts of the world unless coordinated measures are taken to protect people most at risk. Climate change, the mobility of people within and across borders, and the resurgence of the Aedes aegypti mosquito, have combined to increase the likelihood of yellow fever epidemics," WHO says.
Travelers who are infected can spread the virus to new areas if they are bitten by Aedes mosquitoes after they've arrived. That's how Zika has spread across the world, causing outbreaks in Florida and Texas.