CASIMIRO DE ABREU, Brazil >> This small city in the state of Rio de Janeiro is on high alert after authorities confirmed the death of one man by yellow fever and said they were investigating several other possible cases.

Health authorities this week confirmed that 38-year-old Watila Santos died from the illness on March 11.

A neighbor of Santos, Alessandro Valenca Couto, was infected and sent for treatment to a hospital in the city of Rio de Janeiro, where he is recovering.

Authorities are investigating possible cases involving four relatives of Santos, including a 13-year-old and a 9-year-old.

In the city center and rural areas of Casimiro de Abreu, about 93 miles (150 kilometers) from Rio de Janeiro, a large tent has been set up to vaccinate people. Authorities say around 30,000 of the city’s 42,000 people have been vaccinated in recent days.

“I’m really scared,” said Tais da Silva Almeida, a mother of two who arrived Friday to get vaccinated. “If adults can’t deal with the illness, imagine the children.”

Yellow fever is transmitted by mosquito and causes fever, body aches, vomiting and sometimes jaundice. Rio de Janeiro’s state Health Department has announced plans to vaccinate its entire population as a preventative measure. It says it will need 12 million doses to reach a 90 percent vaccination rate by year’s end.

The vaccinations come as cases continue to be confirmed in several areas nationwide. Brazil’s Health Ministry says that at least 424 people have been infected with yellow fever in the largest outbreak the country has seen in years.

Of those, 137 have died, and more than 900 other cases are under investigation. The vast majority of confirmed cases and deaths have been in the southeastern state of Minas Gerais, which borders the state of Rio de Janeiro.

In Casimiro de Abreu, health workers visited houses in rural areas and inspected stagnant water, where mosquitoes lay eggs. The state also sent experts to nearby parks and reserves with monkey populations to monitor the situation with the primates, which are a primary reservoir of yellow fever.

Meanwhile, in a group of houses near a lush jungle area a few miles (5 kilometers) from downtown, relatives of Santos wait for news about the four members of the family who may be infected.

Walace Santos, the younger brother of the man who died, said he took solace in knowing that the death raised alarm bells that could save others.

“Wherever he is now, he knows that because he died a lot of lives were saved,” said Santos.