Protective equipment is in short supply. Here, a Liberian burial team carefully disinfects its gloves before disposing of them. John Moore/Getty Images hide caption

itoggle caption John Moore/Getty Images

A health worker cleans his hands with chlorinated water before entering an Ebola screening tent at the Kenema Government Hospital in Sierra Leone. More than 300 Sierra Leoneans have died of the disease. Michael Duff/AP hide caption

itoggle caption Michael Duff/AP

In 1995, amid an Ebola outbreak, Zairian Red Cross personnel picked up sick people and bodies left on the streets of Kikwit, 250 miles from the capital Kinshasa. Jean-Marc Bouju/AP hide caption

itoggle caption Jean-Marc Bouju/AP

Particles of H5N1 virus — a particularly dangerous type of bird flu that can infect people — attack lung cells. Chris Bjornberg/Science Source hide caption

itoggle caption Chris Bjornberg/Science Source

Human immunodeficiency virus Type 1 inserts its genetic material into the DNA of human cells, turning them into little HIV factories. Eye of Science/Science Source hide caption

itoggle caption Eye of Science/Science Source

Doctors used a rapid DNA test to identify a Wisconsin teen's unusual infection with Leptospira bacteria (yellow), which are common in the tropics. CDC/Rob Weyant hide caption

itoggle caption CDC/Rob Weyant

Red blood cells infected with the Plasmodium falciparum parasite. Plasmodium is the parasite that triggers malaria in people. Gary D. Gaugler/Science Source hide caption

itoggle caption Gary D. Gaugler/Science Source