Ebola disease

Ebola disease, which many people simply call Ebola, is a rare illness, but a severe and often deadly one. It's caused by viruses in the genus Orthoebolavirus, a group of related viruses that belongs to a larger family called Filoviridae.

To see why it's so serious, you have to follow the virus inside the body. It usually gets in through the mucous membranes, the moist linings of the mouth and nose and the surface of the eyes. Once inside, some of the first cells it attacks are mononuclear cells, a type of white blood cell that helps defend the body. When these cells are destroyed, they release cytokines, the immune system's chemical messengers, and reactive oxygen species, unstable molecules made from oxygen. That leads to edema, meaning fluid that builds up in the tissues, and to hypovolemic shock: the blood runs low on volume and can no longer reach the organs properly. Picture a garden hose with too little water in it; no matter how hard the pump pushes, the stream never makes it to the back of the yard. Add the damage caused by the immune response itself, and the result is multiple organ failure, when several organs stop working at the same time.

It spreads through direct contact with body fluids, such as blood, or with objects contaminated by them. And one detail matters: as long as symptoms haven't appeared yet, an infected person doesn't pass it on.