As the world races to nail down the causative agent of Severe Acute Respiratory Syndrome (SARS), no one is firmly nodding at any of the three, possibly four, leading hypotheses.

Is the culprit a paramyxovirus subtype called human metapneumovirus (hMPV), as Health Canada’s National Microbiology Laboratory in Winnipeg proposes? The viruses of the paramyxoviridae family are well-known agents responsible for human respiratory infections, mumps, and measles. Usually, hMPV does not cause respiratory symptoms as severe as those witnessed in the current global outbreak.

Or is the pathogen a coronavirus of the coronaviridae family, as researchers in Toronto and at the U.S. Centers for Disease Control and Prevention suspect? This candidate belongs to a family typically causing colds and upper-respiratory tract infections that are usually contagious.

Perhaps SARS is a combination of both viruses—that’s an idea advanced by researchers in the World Health Organization. Or maybe the agent is something new that jumped from animals to humans.

With today’s molecular biology technologies we will probably catch the causative agent quickly, avoiding the six months that were needed in 1976 to track down the cause of a severe pneumonia called Legionnaire’s disease.

Classification of the pathogen and production of clear-cut diagnostic tools involves several steps: examining a collection of samples at different stages of the illness, culturing specimens both in mice and cells in vitro, and isolating the agent’s genetic material for sequencing and comparing against a genome database. These steps take several days at the very least.

Scientists now think they are close to identifying the cause of SARS. In a paper released by the New England Journal of Medicine on Monday, Toronto researchers pointed to a novel coronavirus as the probable cause, and they identified short genetic sequences from the virus that can be used to test for its presence using basic genetics lab techniques.

But they still aren’t certain this virus is to blame. Settling on a definitive cause and creating a standard test will take more time, and of course it could be years before a vaccine is realized. Coughing up an effective treatment for SARS is not a trivial endeavour.