Malaria is a serious and sometimes fatal disease transmitted by mosquitoes and causes symptoms that typically include fever, fatigue, vomiting and headaches (Other symptoms could be muscle pains, diarrhoea, generally feeling unwell). In severe cases it can cause yellow skin, seizures, coma or death (most deaths are caused by P. falciparum). These signs and symptom usually begin 8–25 days following infection, therefore if you become infected with the most serious type of malaria, there is a risk you could quickly develop severe and life-threatening complications such as breathing problems and organ failure if you are not treated promptly.
When you're bitten by a malaria-infected mosquito, the parasites that cause malaria are released into your blood and infect your liver cells. The parasite reproduces in the liver cells, which then burst open. This allows thousands of new parasites to enter the bloodstream and infect red blood cells. The parasites reproduce again in the blood cells, kill the blood cells, and then move to other uninfected blood cells. Malaria can be a very serious disease for a pregnant woman and her developing fetus. If the infected person is not treated, serious complications or death can occur. Malaria caused by P. falciparum can come back (recur) at irregular intervals for up to 2 years if treatment is not complete. And P. malariae can remain in the blood of an infected person for more than 30 years, usually without causing any symptoms.
One example could be singer Cheryl Cole (article), who was given 24 hours to live after contracting the disease during a trip to Tanzania.
Seek medical advice immediately if you develop symptoms of malaria during or after a visit to an area where the disease is found, even if it is several weeks, months or a year after you return from travelling.