By Mary Hennock BBC News Online business reporter

The firm's building has been sealed

"We prepared for the worst, and the worst actually happened!" laughs John Wu, Chief Technology Officer of Alibaba.com.

Mr Wu's cheeriness seems at odds with the fact that Alibaba.com's entire head-office staff of 400 people have spent the last week in quarantine after a colleague caught Sars.

But nobody else has got sick and the China-based online billboard's business is booming, thanks to the Sars outbreak.

"Since the Sars thing happened our traffic has a 40% growth every week," Alibaba founder and chief executive Jack Ma tells me by mobile phone from his confinement in the east coast city of Hangzhou.

Team spirit

Mr Ma and his staff have all been working from home since the company went into voluntary quarantine on 7 May.

We have a lot of fun these days Jack Ma, Alibaba.com CEO

In the evenings, they hold online Karaoke sessions.

The firm holds its meetings via chatrooms, using Yahoo! instant messenger. "It's very exciting," says Mr Ma.

"Four hundred people is like 400 families...We have a lot of fun these days," he adds. The evening singsongs are especially effective at boosting morale. "People pray and give wishes for our colleague."

"It's a huge experience for our company."

Unlucky visit

Alibaba is a match-making service bringing together foreign and domestic buyers with Chinese suppliers of manufactured goods.

It is benefitting as ever more buyers are trying to source Chinese goods without travelling.

The local government was very supportive Jack Ma, Alibaba.com CEO

The crisis began after an employee returned from a business trip to Guangdong province - the starting point of the Sars outbreak - with a fever.

She was soon hospitalised with suspected Sars.

When Sars was confirmed, health officials issued a compulsory quarantine order on her colleagues and sealed the building, disrupting about 40 mostly small companies.

But Alibaba had opted for voluntary quarantine 12 hours earlier.

Training and planning

"The local government was very supportive on that. We talked to the telephone companies, they arranged within three days (that) every family have got internet connections," says Mr Ma.

Quarantine systems are improving

Most of Alibaba's web-savvy staff already had computers at home, so not much equipment needed moving. Local officials supplied food and twice-daily disinfection visits.

Alibaba's bosses are now congratulating themselves on their foresight in drawing up a disaster response plan back in mid-April.

Perhaps unsurprisingly, they believe that good management has helped them weather the storm.

Based in a small city near the magnetic pull of Shanghai, Alibaba has relied on recruiting young staff and training them intensively, said Mr Wu, who is based in the small California office.

One side-effect of constant training is strong team spirit, he told a China-Britain Business Council conference in London.

He says his title should be chief training officer; his CEO makes the same joke - chief education officer in his case.

Alibaba has been smart, but it has also been lucky. Its sick staff member has recovered without infecting anyone.

Nor does Sars appear to have taken hold in Hangzhou, which has had only a handful of cases.

The firm heard on 14 May it was to be released from quarantine: "We are all set free," says Mr Ma.

Sars windfall

Alibaba was founded in 1998 by Mr Ma, formerly an official at China's foreign trade ministry.

Two thirds of its fee-paying users are China-based, while the rest of its business comes from abroad.

And it seems to be having a Sars windfall. Page hits have risen from about 4,000 a day at the end of last year to 12,000 a day, yielding first quarter revenues of $5.5m (�3.4m) and positive cash flow of $2.2m.

Alibaba's staff will carry on working from home for the rest of the week. Some of them have grown to like it, says Mr Wu.