Malaria

Malaria is one of the most widespread diseases in the world. Some 300 million people are infected every year of which some 1.5 million die.
It's caused by protozoans of genus Plasmodium.

They are spread by mosquitoes of genus Anopheles. These mosquitoes breed in water (e.g. swamps).
The Plasmodium attacks red blood cells, multiplies there and goes to another red blood cells. The disease therefore causes fever and anemia, among other things. However, the symptoms quickly pass away after killing of the parasite.
In some endemic countries, the disease is so widespread that a lot of people develop immunity.  Also, malaria caused a great evolution pressure on human genome. In endemic ideas, people have mutations causing resistance to malaria even though those have other negative effects (e.g. sickle-cell anemia)
Children and pregnant women are the most vulnerable groups.