In January 2003, a previously well middle-aged man from Guangdong Province in the People' s Republic of China developed a cough, fever, and aching muscles - typical symptoms for any one of 100 flu-like illnesses. But this illness took an atypical course. The man collapsed two days later and was admitted to hospital with suspected pneumonia.

Symptom anomalies

Unusually though, in a patient with pneumonia, doctors detected few abnormal sounds from his lungs. Another odd feature was the absence of typical viral symptoms - he didn' t have a runny nose, sneezing or sore throat, nor did his cough produce much sputum. A chest X-ray showed dramatic pneumonia-like shadowing. Blood tests showed suppression of his white blood cell count and low plasma proteins, indicating severe infection with a very virulent organism. The oxygen concentration in the man' s blood was perilously low.

As with any case of severe pneumonia, the patient' s sputum was was first sent to investigate the crisis. By early April 2003, less than five months after the first suspected case of SARS and three months after the first death, doctors in Hong Kong had smashed the world record for isolating a new virus. They grew the organism from the body fluids of affected patients and proved (by injecting it into monkeys) that it was the cause of the deadly SARS disease. They sequenced its genetic code. The culprit was a tiny, crown-shaped (' corona' ) virus around 30 nanometers in diameter, which has been tentatively dubbed Human Pneumonia-Associated Coronavirus (HPAV). For the moment, SARS is still non-preventable and incurable.

I don' t know about you, but I have a business trip to Australia booked for next month that involves a stopover in Hong Kong. Should I cancel my trip, or is the ' epidemic' a case of mass panic and media hype?

Without doubt, SARS is a real epidemic (strictly a pandemic - that is, an epidemic that has spread across several continents), but that in itself is not a reason to hit the bunker. More people have died of asthma in the last month than have died of SARS. The question to ask is not:' Is SARS spreading?' but ' How likely am I to (a) catch it and (b) die of it?'

The likelihood of catching SARS from someone who has it is known as the contagiousness of the disease. A disease can be very contagious but not serious (such as the common cold), serious but not very contagious (such as BSE), or both serious and contagious (such as hepatitis B). You know that if someone brings their cold to the office and coughs vigorously, most people will go down with the same cold. SARS is less contagious than that. If someone was brewing it and coughed, most people