Yellow fever is found only in sub-Saharan Africa and tropical South America, and is almost always transmitted by mosquitoes. Yellow fever can cause flu-like symptoms, hepatitis and hemorrhagic fever. Death can occur in children and people with compromised immune systems. Fatality rates stay constant at about 20%. A vaccine is available; however side effects can be as dangerous as the fever itself. In milder reactions to the vaccine the patient may suffer from rash, headache, and hypersensitivity. Major reactions to the vaccine include fever, cognitive impairment and even multiple organ failure.