A "new" strain of deadly bird flu dubbed "Disease X" by the World Health Organization (WHO) has killed hundreds of people in China, and is just three mutations away from becoming transmissible between humans, according to experts.

The strain, H7N9, circulates in poultry and has killed 623 people out of 1,625 infected in China - a mortality rate of 38.3%. While first identified in China in 2013, H7N9 has recently emerged as a serious threat seemingly overnight.

Professor Jonathan Van-Tam, deputy chief medical officer for the UK, told The Telegraph that H7N9 could cause a global outbreak.

“[H7N9] is an example of another virus which has proven its ability to transmit from birds to humans," said Van-Tam, who added "It’s possible that it could be the cause of the next pandemic."

The WHO says N7N9 is "an unusually dangerous virus for humans," and "one of the most lethal influenza viruses that we've seen so far"

“H7N9 viruses have several features typically associated with human influenza viruses and therefore possess pandemic potential and need to be monitored closely,” said Dr. Yoshihiro Kawaoka of the University of Wisconsin-Madison.

Researchers led by James Paulson of the Scripps Research Institute in California have been studying the mutations which could potentially occur in H7N9's genome to allow for human-to-human infection.

The team’s findings, published in the journal PLoS Pathogens on Thursday, showed that in laboratory tests, mutations in three amino acids made the virus more able to bind to human cells — suggesting these changes are key to making the virus more dangerous to people. -Japan Times

That said, the mutations would need to occur relatively close to each other to become more virulent, which has a low probability of happening according to Fiona Culley, an expert in respiratory immunology at Imperial College London.

"Some of the individual mutations have been seen naturally … these combinations of mutations have not,” and added: "The chances of all three occurring together is relatively low."

Wenday Barclay, a virologist and flu specialist also at Imperial College says the study's findings reinforce the need to keep the H7N9 bird flu under close surveillance.

“These studies keep H7N9 virus high on the list of viruses we should be concerned about,” she said. “The more people infected, the higher the chance that the lethal combination of mutations could occur.”

According to the CDC, Human infections with bird flu viruses can happen when enough virus gets into a person’s eyes, nose or mouth, or is inhaled. This can happen when virus is in the air (in droplets or possibly dust) and a person breathes it in, or when a person touches something that has virus on it then touches their mouth, eyes or nose.

The reported signs and symptoms of avian influenza A virus infections in humans have ranged from mild to severe and included conjunctivitis, influenza-like illness (e.g., fever, cough, sore throat, muscle aches) sometimes accompanied by nausea, abdominal pain, diarrhea, and vomiting, severe respiratory illness (e.g., shortness of breath, difficulty breathing, pneumonia, acute respiratory distress, viral pneumonia, respiratory failure ), neurologic changes (altered mental status, seizures), and the involvement of other organ systems -CDC

Rare human infections with some avian viruses have occurred most often after unprotected contact with infected birds or surfaces contaminated with avian influenza viruses. However, some infections have been identified where direct contact was not known to have occurred. Illness in people has ranged from mild to severe.

Don't let this happen to you: