It came as no surprise on Thursday when the World Health Organization declared that the swine flu outbreak had become a pandemic.

The disease has reached 74 countries, and probably met the technical definition of a pandemic  or global spread  weeks ago. Nearly 30,000 cases have been reported, but disease experts think hundreds of thousands or millions of people have actually been infected.

So the agency made official what had become obvious: that the H1N1 virus is spreading quickly in different parts of the world, and its chief, Dr. Margaret Chan, said, “Further spread is considered inevitable.”

The announcement does not mean that the illness, which has been mild in most people, has become any worse. The term pandemic reflects only the geographic spread of a new disease, not its severity. Pandemics typically infect about a third of the world in a year or two, and sometimes strike in successive waves.