Brazil is preparing to fight another mosquito-borne disease, but it’s not Zika. This time around, it’s yellow fever. 

The last outbreak in the US of this rarely-publicized illness occurred in New Orleans over 100 years ago. Yet the disease still ravages regions in Africa, South and Central America. 

Each year, yellow fever kills 45,000 people, primarily in Africa. Another 127,000 become sick with fevers, chills, aches, and pain according to WHO. 

Recently, Brazil has been battling pocketed outbreaks. 

Read More: The Biggest Health Crisis of 2016 Is One You Haven’t Heard About

Brazil reported nearly 500 cases of yellow fever and 162 confirmed deaths resulting from the disease since January 2017. 

The World Health Organization just deployed 3.5 million vaccines from their emergency stockpile to combat the deadly illness in Brazil at the request of the government. 

The vaccines came through the International Coordinating Group on Vaccine Provision which includes WHO, the United Nations Children’s Fund, Doctors Without Borders, and the International Federation of Red Cross and Red Crescent Societies. 

The additional vaccines come in addition to the 18.8 million already distributed in high-risk areas since January. WHO is also helping by sending 15 global experts to assist with technical support and manage the outbreak. 

Cases are currently dominant in four regions— Minas Gerais, Espírito Santo, Rio de Janeiro and São Paulo—and have been linked to two jungle species of mosquitoes, according to the WHO and the Pan American Health Organization.  

Right now, the Brazilian government hopes a vaccination campaign will prevent the spread of yellow fever to Aedes aegypti, the urban species of mosquito common in Brazilian cities where it could rapidly spread in the more densely-populated urban areas. 

Read More: The Zika Virus Is No Longer an Emergency, According to World Health Organization

While Brazil is taking all steps to prevent the spread of yellow fever, the campaign raises a second set of concerns for combatting the disease: vaccine shortage. 

Only about 80 million vaccines are made each year for yellow fever. However, 105 million are needed to meet demands of the growing disease. 

Yellow fever vaccines cost only $1 per dose, but the process of making the vaccine is so cumbersome that global health organizations still have limited supplies. 

To make a yellow fever vaccine, live incubation is required. This entails injecting the vaccine virus into a chicken embryo, letting that sit in a partial egg shell for four days, then blending the solution. The process is 80 years old and makes quick mass production impossible with the current infrastructure. 

Last year, WHO used 30 million emergency supplies to quell yellow fever outbreaks in Angola and the Democratic Republic of Congo. 

Countries also donated back their rations of yellow fever vaccines in 2016. They did this so that WHO could dilute vaccines, creating enough to fight outbreaks, and prevent an epidemic. 

As yellow fever rears its ugly head, the need for the vaccine appears to be just as in demand this year as it was in the past.