HONG KONG - Someone once mused to me that one doesn't truly know Hong Kong until one understands what it went through during the Sars crisis in 2003.

Indeed, a stranger to the city might wonder:

- Why are there so many people wearing face masks, out on the streets and indoors too?

- Why are toilets so clean, with even standalone public toilets in the middle of nowhere outfitted with automatic soap dispensers?

- Why are there so mainland tourists here?

- Why do people approach a certain bubbly soda with trepidation?

Random questions.

But the answers are all related to those 1,000 days of panic in 2003, between February and May, which saw a once-confident city brought to its knees by a mystery virus.

Sars was first transported over by a Guangdong doctor on Feb 21. From a hotel in Mongkok, the virus raced throughout Hong Kong, and eventually the world, infecting 8,096 people and killing 774 of them.

Hong Kong was the hardest hit, with 1,755 people infected and 299 deaths.

The sheer panic of those three months before the World Health Organisation declared the epidemic contained, was to leave an indelible scar on Hong Kong - one still visible a decade on.

I experienced it first-hand myself some time last year. While taking the lift at the Times Square mall in Causeway Bay, something tickled my throat. I did something unspeakable. I coughed.

Immediately, a young woman next to me bolted to the far end of the lift. Her mother put her arm around her protectively. The rest of the passengers glared at me balefully.

It was a very uncomfortable 10-second ride to my destination.

So while recent surveys have found that Hong Kongers' personal hygiene standards have slipped in the past decade, the fear still clearly reverberates.

The use of face masks is prevalent here - not so much to protect oneself from the pollution outdoors, but to protect others from germs if one is not well.

Toilets are assidiously maintained by a battalion of cleaners. A masked je je (how Hong Kongers call their "aunties") - even out in the country parks in the New Territories - is on duty and will be cleaning up after you.

The influx of tourists from the mainland and the resulting social and economic impact was a direct result of Sars too. The Hong Kong economy was moribund in the wake of the crisis. The hitherto thriving tourism industry - which was responsible for the quick dissemination of the virus across borders - in particular, was affected, as people shied away.

To help revive the city's economy, Beijing started a scheme in July that year, allowing mainland residents to visit Hong Kong and Macau on an individual basis. Before, they could do so only if they had obtained business visas, or are part of group tours. That marked the beginning of the explosion of Chinese visitors here - for better or for worse.

As for the soda - well, is there any wonder that Sarsi never took off here?