HONG KONG – When word came in April that an entirely new, highly infectious disease–swine flu–was spreading beyond Mexico, this was the most paranoid city in the world. Land at the airport with a fever and runny nose, and you’d risk being quarantined for a week, just in case you’d brought the new disease with you.

People in this city, scarred by the SARS epidemic, still shudder when they hear someone cough. For Hong Kongers, the sound brings back memories of the scary time when the city nearly shut down and residents feared death from a new mystery disease. Schools were closed. When people left their homes–which wasn’t often–many wore medical masks to reduce their exposure to anyone who might be sick.

Fast-forward. If you catch the flu in Hong Kong today–or in most places–you won’t be rushed to the isolation ward just in case it proves to be the new swine flu. You’ll be told to go home and rest and not cough on anyone. “Everyone has finally realized that this is going to spread,” said Dr. Anthony Mounts, a flu specialist at the World Health Organization.

What a change.

That’s because SARS and swine flu are proving such opposites. Both are new diseases, which means that none of the 6 billion people on the planet had immunity to them when they came on the scene. The World Health Organization feared that if they developed into full-blown pandemics and raced around the world, millions would die, defenseless against new strains.

SARS had a relatively high mortality rate, but it just so happened that it wasn’t easy to catch. Swine flu is the opposite: easy to catch, like any seasonal flu, but with a mortality rate no higher than seasonal flu for most people.

But doctors expect many more people will die of swine flu than regular flu because more people will catch it. In a normal year, 8% to 10% of the population comes down with the flu, and a fraction of those sickened by flu die from it. With a new pandemic flu, like swine flu, experts expect at least three times as many people to catch it because of the lack of exposure to the new disease, said Dr. Mounts, the WHO medical officer.

But with a third of the world population likely to get swine flu, even a small mortality rate is amplified. After all, even during the notorious 1918 flu outbreak, the mortality rate was merely 2%–meaning that 98% of those who got the flu survived. The WHO doesn’t have a reliable estimate for the mortality rate of swine flu, but it appears to be similar to that of a mild seasonal flu, except for high-risk groups like pregnant women and those who are already sick with other ailments.

That’s lucky, since swine flu is now spreading across the globe uncontrollably.

Worldwide, the World Health Organization has confirmed 162,000 cases of swine flu, a number officials concede is an undercount because many governments have stopped testing and tracking all but the most severe cases.

Like other governments around the world, Hong Kong–where there are now more than 4,500 confirmed cases, up by 232 from 24 hours before–has stopped trying to prevent the flu, and has switched to slowing down its spread. Essentially, the new swine flu management strategy is for doctors and governments to try to buy time until a swine flu vaccine, expected in the fall, arrives.

“Our strategy is to slow down the spread of the disease as well as minimizing complications and fatality through promulgation of personal hygiene practices and other mitigation measures, and wait for the vaccine to be available in the coming months,” said Dr. Thomas Chung, spokesman for the Hong Kong Center for Health Protection.

But flu viruses constantly mutate, sometimes growing more deadly, sometimes less. A cluster of swine flu cases resistant to Tamiflu, the most popular treatment, recently developed along the U.S.-Mexican border.

Some doctors say that catching swine flu now, while it is mild, could prevent a more serious infection later by building immunity in those sickened by the new flu virus.

“In the U.K. and Europe and America, it is spreading so fast that it is very likely that most people will be exposed to or get swine flu, and this will help them develop immunity before the second wave,” said Dr. Raymond Ng, a doctor with OT and P, a family practice in Hong Kong. That second wave could be a mutated, more lethal flu. “Having some immunity gives us some protection against this virus,” Dr. Ng said.