Chikungunya
Some scientists think that chikungunya has become much more common because of a change in the virus's genetic code.  This change may have made it easier for the virus to make copies of itself in mosquito cells.  It may have also allowed the virus to be spread more easily by the Asian tiger mosquito (Aedes albopictus).  This is important because the Asian tiger mosquito lives in many more places than Aedes aegypti, the other species of mosquito that spreads chikungunya to humans.  Aedes aegypti lives only in tropical places.  However, the Asian tiger mosquito spreads easily, and lives in Europe, the Americas, the Caribbean, Africa, and the Middle East.