The World Health Organisation (WHO) has issued a global alert about the emergence of a new virus that was previously unknown in humans and can cause a potentially fatal acute respiratory infection.

Here is an at-a-glance guide to the virus:

The virus belongs to a family called coronaviruses and has so far been confirmed in only two cases globally. Both occurred between July and September 2012.

The first case was in a 60-year-old man in Saudi Arabia and proved fatal. The second is in a 49-year-old Qatari man who recently visited Saudi Arabia. He had the infection diagnosed after travelling to London in early September.

The WHO has not yet given the virus a name, but scientists at Britain's Health Protection Agency (HPA) refer to it as "London1_novel CoV 2012".

Human coronaviruses were first identified in the mid-1960s and are named for the crown-like projections on the surface of the virus.

The family includes viruses that cause the common cold and SARS, or Severe Acute Respiratory Syndrome, which emerged in China in 2002 and killed about 10 percent of approximately 8,000 people infected worldwide.

Symptoms in the two confirmed cases include fever, cough and breathing difficulties. It is not yet clear whether these are typical or whether the virus could be circulating more widely but causing milder illness.

Coronaviruses are typically spread like other respiratory infections, such as flu, travelling in airborne droplets when an infected person coughs or sneezes.

At this stage it is not clear where the virus came from. New infections can emerge from mutations to existing viruses, or be caught from infections circulating in the animal population. Human diseases that come from animal are called zoonoses.