Causes and Risk Factors of Yellow Fever

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Yellow fever is a potentially deadly disease caused by a type of virus called a Flavivirus. People generally come in contact with this virus through mosquito bites, and it's most common in Africa, Central America, and South America. However, outbreaks can happen anywhere in the world. They're especially likely in areas with a large mosquito population.

Not everyone who's bitten by an infected mosquito will get sick, though. Only some groups of people are more likely to contract a severe form of the illness.

yellow fever risk factors

Common Causes

While mosquito bites are the most common cause of yellow fever, they're not the only cause. It's also possible to catch yellow fever if you're bitten by an infected primate or human. Of course, people and primates are far less likely to bite than the mosquito, so an infected animal doesn't pose near as much of a threat.

Other biting animals and insects aren't a threat because only humans, primates, and mosquitoes are known hosts of the virus.

Not all mosquitoes carry the yellow fever virus—only a few mosquito species are known to carry it. Furthermore, those mosquitoes only pose a threat if they've previously bitten an infected person or animal.

After the virus goes into the bug's bloodstream, it ends up in its salivary glands. When mosquitoes bite us, their saliva carries it into our blood.

Disease Spread

Yellow fever is not spread directly from one person to another, not even through close contact—it takes some kind of bite to get the virus directly into your bloodstream.

Typically, outbreaks in urban areas start with someone who's visited a jungle in Africa, Central America, or South America. In those regions, yellow fever is endemic over 40 countries, where it's believed that the monkey population is widely infected. Sub-Saharan Africa is home to about 90 percent of reported cases every year.

The World Health Organization (WHO) estimates that, worldwide, we see up to 170,000 reported cases of yellow fever every year. At least 30,000 people die from it annually.

Those are only the reported cases, though. We can't say how many people come down with mild cases because it's generally just the severe ones that are reported. One study published in 2014 estimated that somewhere between one and 70 people are mildly infected for every severe case reported.

Because an infected person doesn't start having symptoms for a few days, they're often unaware that they're sick when they travel back home. Then they're able to spread the virus to uninfected mosquitoes starting a little before the fever hits and for about three to five days afterward. This can lead to outbreaks. It's possible for outbreaks to lead to epidemics.

4 Needed Factors for Outbreaks to Occur

According to the World Health Organization, certain conditions have to be met for an outbreak to occur. The region the infected person is in must have:

  • Mosquito species that are able to transmit it
  • Specific climactic conditions (i.e., tropical rainforests, high humidity, bodies of still water such as lakes)
  • A large primate population to sustain it
  • A large population of unvaccinated people


Certain people may be more likely to die from yellow fever than others based on their genetics.

A 2014 study published in the journal mBio reports that during 19th-century outbreaks in the United States, death was nearly seven times more likely in Caucasians (white people) than in non-Caucasians. They speculated that the difference was due to genetic differences in certain aspects of the immune system.

Lifestyle Risk Factors

The biggest risk factor for yellow fever is living in or traveling to regions where yellow fever is common. However, that risk can be greatly reduced by being vaccinated. Some countries where the disease is endemic won't allow people to enter without proof that they've had the vaccine.

Babies and people over 50 are more likely to develop severe cases and to die from yellow fever.

However, proper prevention greatly lowers the risk of contracting the disease. For those who do become infected and have severe symptoms, prompt medical attention is crucial.

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  1. World Health Organization. Yellow Fever: Fact Sheet. Updated May 7, 2019.

  2. Johansson MA, Vasconcelos PF, Staples JE. The whole iceberg: estimating the incidence of yellow fever virus infection from the number of severe cases. Transactions of the Royal Society of Tropical Medicine and Hygiene. 2014 Aug;108(8):482-7. doi:10.1093/trstmh/tru092

  3. Centers for Disease Control and Prevention. Transmission of Yellow Fever Virus. Updated January 15, 2019.

  4. Centers for Disease Control and Prevention. Yellow Fever Maps. Updated January 15, 2019.

  5. Blake LE, Garcia-Blanco MA. Human genetic variation and yellow fever mortality during 19th century U.S. epidemics. mBio. 2014 Jun 3;5(3):e01253-14. doi:10.1128/mBio.01253-14

  6. Centers for Disease and Prevention. Yellow Fever Vaccine. Updated May 16, 2019.

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