Zika virus occurs in tropical areas with large mosquito populations, and is known to circulate in Africa, the Americas, Southern Asia and Western Pacific.

People catch Zika virus by being bitten by an infected Aedes mosquito – the same type of mosquito that spreads dengue, chikungunya and yellow fever.

Mosquitoes and their breeding sites pose a significant risk factor for Zika virus infection.

Yellow fever is a viral disease, found in tropical regions of Africa and the Americas. The "yellow" in the name refers to the jaundice that affects some patients. It principally affects humans and monkeys, and is transmitted via the bite of Aedes mosquitoes. It can produce devastating outbreaks, which can be prevented and controlled by mass vaccination campaigns.

Large scale outbreaks occur every 3 to 10 years in villages or cities in the absence of large scale immunization. Sporadic cases can occur regularly in endemic areas.