The UN World Health Organization is urging travelers to Rio De Janeiro and Sao Paulo to get vaccinated against Yellow Fever at least ten days prior to their trip, as Brazil faces its worst outbreak of the disease in 40 years.

Yellow fever is an acute viral hemorrhagic disease transmitted by infected mosquitoes.  The "yellow" in the name refers to the jaundice that affects some patients.  Currently, it's being transmitted by two types of mosquitoes - the Haemagogus or Sabethes.  But if it reaches one of the big urban areas, there are fears it could make the jump to the Aedes egypti which would drastically increase the chances of human infections.  So far, that hasn't happened.

The problem is that not enough people are vaccinated.  There are nearly 600 confirmed cases in human beings, mostly in two southeastern states.  But another 450 cases are being investigated and the outbreak that started in Minas Gerais state appears to be marching towards the Atlantic Ocean coast into areas where it hasn't been before.  More than 200 people have died from it in Brazil since December 2016.

The virus is also spread in animals:  At least one monkey died at the zoo in Goiania, an inland city; several suspected epizootic cases of Yellow Fever are currently under investigation in Bahia state along the northern coastal area, including the urban area of Salvador.