Rat traps are a weapon behind used to fight the plague in Madagascar, since the rodents carry the disease. But getting rid of all the rats would be difficult — and without rats, plague-infected fleas could then turn to humans for a blood meal. RIJASOLO/AFP/Getty Images hide caption
Workers spray to kill fleas in a public school in Antananarivo, Madagascar's capital. A bite from an infected flea can spread the plague, which has stricken 157 people in the island nation since August. Anadolu Agency/Getty Images hide caption
Twins Heloisa (left) and Heloa Barbosa, both born with microcephaly, had a one-year birthday party on April 16 in Areia, Brazil. Their mother says she contracted the Zika virus during pregnancy. Mario Tama/Getty Images hide caption
This computer-generated image shows the structure of the hepatitis B virus. Science Picture Co/Science Picture Co hide caption
A newly detected type of Klebsiella pneumoniae bacteria (in red) has acquired genes that make it more deadly. Science Source hide caption
A worker fogs a neighborhood on the outskirts of Colombo in Sri Lanka in an effort to ward off mosquitoes. The country is facing an outbreak of dengue fever, a mosquito-borne disease. Ishara S. Kodikara/AFP/Getty Images hide caption
Maria Carolina Silva Flor and Joselito Alves dos Santos with their 18-month-old daughter, Maria Gabriela Silva Alves. The baby was born with Zika syndrome. Amanda Klasing hide caption
Ticks of the genus Hyalomma, pictured here, are principally responsible for passing on Crimea-Congo hemorrhagic fever to humans. Adam Cuerden hide caption
The scene after a mortar attack on the southern city of Taez. Ahmad Al-Basha/Getty Images hide caption
Nurses give the oral polio vaccine to a Syrian child in a refugee camp in Turkey. The oral polio vaccine used throughout most of the developing world contains a form of the virus that has been weakened in the laboratory. But it's still a live virus. Carsten Koall/Getty Images hide caption
A tuberculosis patient in a hospital for chest diseases in the Indian region of Kashmir. NurPhoto/Getty Images hide caption
Brazilian members of the United Nations Stabilization Mission In Haiti stand by a shelter in Les Cayes ahead of a visit by U.N. Secretary-General Ban Ki-moon on October 15, 2016. Hector Retamal/AFP/Getty Images hide caption