Since the beginning of the year, Nigeria has been gripped by an outbreak of a deadly disease. Lassa fever is one of a number of illnesses which can cause dangerous epidemics, but for which no vaccine currently exists.
Lassa fever is not a new disease, but the current outbreak is unprecedented, spreading faster and further than ever before.
Health workers are overstretched, and a number have themselves become infected and died.
The potentially fatal disease is a so-called "viral haemorrhagic fever", which can affect many organs, and damage the body's blood vessels.
But it is difficult to treat.