Obesity is a disease where people accumulate more and more fat. When they reach a certain point, their fat stops working and they develop disease, such as type 2 diabetes. But not all fat is bad. The fat that accumulates in obesity is called white fat, but a second form of fat (brown fat) could actually be used to treat obesity.
Brown fat has evolved to turn fuel into heat. In small animals, like mice and voles, brown fat makes heat that helps them survive, even in freezing temperatures.