Gerald Pier, of Harvard Medical School in Boston, and his colleagues, think they have the answer: typhoid fever.

ECONOMIST: Cystic fibrosis and typhoid fever: A Faustian bargain | The

The vaccine provides protection for hepatitis A for patients over 15 years for a minimum of 12 months and protection against typhoid fever for up to three years.

BBC: Combined jab launched for holidaymakers

Their research, published in this week's Nature, studied the relationship between Salmonella typhi, the bacterium that causes typhoid fever, and the protein which, when faulty, causes cystic fibrosis.

ECONOMIST: Cystic fibrosis and typhoid fever

The RPS said: "We would be very concerned if a patient took a homeopathic preparation to travel to an area where there may be yellow fever, typhoid, malaria, believing they were safe when in fact they wouldn't be safe".

BBC: Cases dropped against malaria homeopaths

He was flown by air ambulance from Freetown, the capital of Sierra Leone, earlier this month with suspected malaria or typhoid, but was finally diagnosed as having Lassa fever after a battery of specialist tests.

BBC: Tropical virus man 'fighting for life'