World Health Organisation officials say it is likely a new coronavirus that has killed at least 18 people in the Middle East and Europe could be passed between humans.
A virus from the same family triggered the outbreak of Severe Acute Respiratory Syndrome that swept the world after emerging in Asia and killed 775 people in 2003.
French authorities announced today that a second man had been diagnosed with the disease after sharing a hospital room with France's only other sufferer.
WHO Assistant Director-General Keiji Fukuda said there was no evidence so far the virus was able to sustain "generalised transmission in communities" - a scenario that would raise the spectre of a pandemic.
But he added: "Of most concern ... is the fact that the different clusters seen in multiple countries ... increasingly support the hypothesis that when there is close contact, this novel coronavirus can transmit from person to person.
He said there is a need for countries to increase levels of awareness.
A public health expert, who declined to be identified, said "close contact" meant being in the same small, enclosed space with an infected person for a prolonged period.
The virus first emerged in the Gulf last year, but deaths have also been recorded in Britain and France of people who had recently been in the Middle East.
A total of 34 cases worldwide have been confirmed by blood tests so far.