UK health officials have issued an alert after a person with Lassa fever, which can cause an Ebola-like illness, travelled to England at the end of last month.
The UK Health Security Agency (UKHSA) said it had been informed under international rules that a person travelled to England from Nigeria while they were unwell with Lassa fever.
They then returned to Nigeria where they were diagnosed, it said.
Officials said they were "working to identify people who were in contact with the affected individual while they were in the country".
Lassa fever does not spread easily between people and the overall risk to the public is regarded as "very low".
The virus, which is endemic in parts of West Africa, particularly Guinea, Liberia, Nigeria and Sierra Leone, is carried by a particular type of rodent, called the Mastomys rodent.
The most common way people catch the virus in these countries is through eating contaminated food or breathing in the virus.
Some people can also be infected from the droppings of infected rodents, such as on floors, surfaces, or in food or water.
Some people eat Mastomys, and they may catch Lassa virus this way.
While the virus does not spread easily between people, it can be transferred through blood, saliva, urine or semen.
Most people who get Lassa fever have mild symptoms such as fever, feeling weak, headache and sore throat, and recover.
More serious symptoms include bleeding, difficulty breathing, vomiting, facial swelling, pain in the chest, back and stomach.
Dr Meera Chand, deputy director at UKHSA, said: "Our health protection teams are working at pace to get in touch with people who were in contact with this individual while they were in England, to ensure they seek appropriate medical care and testing should they develop any symptoms.
"The infection does not spread easily between people, and the overall risk to the UK population is very low."
Symptoms usually occur between one and three weeks after having contact with Lassa virus.
