A large outbreak of Lassa fever in Nigeria has people worried. The area is observing an unusually high number of cases this year. As of February 18, there have been 913 cases and 73 deaths, compared to 733 cases and 71 deaths in all of 2017.

Like Ebola, Lassa fever is a hemorrhagic fever, though considered less serious than Ebola. According the WHO, Lassa fever usually starts with a fever and progresses to a headache, muscle pain, nausea, vomiting, and diarrhea. In severe cases there may fluid in the lungs, and bleeding from the mouth, nose or other areas. In the most advanced stage of the disease shock, seizures, and coma may occur. In fatal cases, death usually occurs within 14 days of the onset. The drug Ribavirin, given via IV, is considered an effective treatment for Lassa fever if given within 6 days of the onset of symptoms.

What’s causing such a large outbreak? According to Dr. Chikwe Ihekweazu, director of the Nigeria Centre for Disease Control, a couple things may be at play: Improved detection, and that Nigeria’s growing population has brought people closer to the disease host: the infamous rat.

According to NPR:

“West Africa’s dry winters push rodents closer to people to scavenge for food. Virus-carrying rats may defecate or urinate in grains and other food; people can pick up the virus from contact with contaminated products. The virus can also spread between people via bodily fluids. And there are a lot of rats – which means there’s a lot of potential for outbreaks.”

The WHO is scaling up its response to the outbreak, and heath officials are urging people to keep food in sealed containers, as well as limit the proximity of garbage to homes.