An outbreak of yellow fever has killed over 100 people in southern Brazil since the beginning of the year. A massive vaccination campaign in Rio de Janeiro is expected to turn even larger as people rush to take their preventive shots.

Hundreds of people have been lining up in Rio’s public health clinics to receive yellow fever vaccines.

They are afraid of contracting a disease that had been almost fully eradicated -- but is back and causing alarm.

Presently, the outbreak has been limited to rural areas in Brazil’s southeast, where the virus has been spreading to humans by mosquitoes that bit infected monkeys.

Yellow fever is deadly for monkeys and can also be for unvaccinated humans.

A mass vaccination campaign in Rio was announced earlier this month as a preventive measure, but after two cases were confirmed in a rural area of the state, including one death, the campaign was expanded as the demand for the vaccine soars.

Most Cariocas, as Rio’s citizens are called, were never vaccinated.

Brazil’s Ministry of Health says there are more than 400 confirmed cases and 137 deaths in neighboring states.

According to the World Health Organization, a small proportion of patients who contract yellow fever develop severe symptoms and half of those die within seven to ten days.

While Rio’s residents are now desperately trying to get vaccinated, the state government is aiming to vaccinate the whole state population of 12 million by the end of the year or earlier.