Health officials have been contacting British passengers who were on the same flights as a health care worker who has been diagnosed with the potentially deadly Mers virus.
While the risk of infection is very low, the Mers virus kills around a third of people infected and has no cure.
First identified in Saudi Arabia in 2012, Mers (Middle East Respiratory Syndrome) is a severe pneumonia-like respiratory disease caused by a virus.
The main symptoms are a high fever, coughing or shortness of breath and diarrhoea.
It belongs to the coronavirus family that includes the common cold and Sars (Severe Acute Respiratory Syndrome), which killed nearly 800 people across the world in 2003 before being stopped.
While largely existing in the Middle East, cases of Mers have been recorded in Europe, Asia and now the US with the arrival of the healthcare worker in Chicago from Saudi Arabia, via London.
Unlike Sars, though, the Mers virus does not appear to spread easily among groups of people.
Instead those infected have all been working in a health care facility or living in a family environment already affected by the virus.
The World Health Organisation reports it has caused 93 deaths from 250 confirmed cases since 2012, though equivalent figures in Saudi Arabia alone are reportedly higher.
The Mers virus has been found in camels but health officials are yet to explain how it has spread to humans.
The current incubation period - the time after exposure when symptoms may develop - is 14 days, with any illness after that point considered to be unrelated to the virus.