The real danger is more distant: the possibility that sooner or later someone will be simultaneously infected with an avian flu virus and a human flu virus, allowing the viruses to swap genes in a way that makes the bird virus highly transmissible among humans, who would have no natural immunity.

If the virus spreads among these birds, their human owners could be exposed, thus increasing the probability that a person could become simultaneously infected with bird flu and human flu.

In contrast, people who are simultaneously infected with tuberculosis and the human immunodeficiency virus, which causes AIDS, develop active TB at a rate of nearly 10 percent a year.

The laboratory work, however, seems to run counter to what occurs when a person is simultaneously infected by both strains of the virus.

For genetic reassortment to occur, a cell must be simultaneously infected with more than one influenza virus of more than one animal strain.

This variability is compounded when a single cell is simultaneously infected by two or more different strains of HIV.

The women converged on the table, behaving as if they had been suddenly and simultaneously infected with the same benign fever, one that brought color to their cheeks and a glitter to their eyes.

There have also been cases of people who were simultaneously infected with the Lyme disease bacteria and the organism that causes H.G.E., which complicates making an accurate diagnosis, experts said.

When pigs become simultaneously infected with more than one virus, the viruses can swap genes, producing new variants which can pass to humans and sometimes spread amongst them.