There is no risk to the wider public, say authorities, and none of the man's close contacts have caught the virus.

It is unclear how he got it, although there have been cases in some poultry farms in Mexico.

Infections like this can sometime pass from animals to humans.

The man had other underlying health issues that likely made him vulnerable.

He had been bedridden for weeks before getting sick with fever and shortness of breath from bird flu, according to officials.

There is no evidence that the virus could start spreading between people and cause an outbreak, they say.

The World Health Organization and other authorities monitor strains of flu, like this one, that could have the potential mutate and become a threat.

There are many types or strains of avian or bird flu.

As well as birds, some wild mammals - such as seals, otters, wild dogs and foxes - can catch them too.

Cases do occur in people from time to time and patients usually get better.

One type of bird flu, called H5N1, has been spreading for weeks among dairy cow herds in the US, with a small number of cases reported among humans.

The UK recently declared itself bird-flu-free after recording no new cases of H5N1, following outbreaks in poultry and other captive birds.

Responding to the news from Mexico, Prof Ian Brown, Avian Virology Group Lead, Pirbright Institute, said: "The prompt follow up in healthcare professionals and family members in contact with the infected patient provides reassurance at present this is an isolated case."

Dr Ed Hutchinson from the University of Glasgow said it looked likely that the man's infection was a spillover event - meaning he had probably caught it from an infected animal somehow.

"At the moment surveillance is taking place, including testing people who may have been exposed to the virus but fought off the infection to see if they show any signs of an immune response. If there are more human infections with this virus it would become of wider concern.”