As America faces its largest outbreak yet of illness caused by West Nile virus -- 251 cases so far this year, and as many as 1,000 expected -- inevitable questions arise. Why here? Why now? Until 1999, the disease had never even been detected in North America.

No one knows how the virus came to the United States. But it made itself at home, and by 2001 had infected 29 species of mosquitoes, 100 species of birds and many mammals, including humans. It has now reached 36 states and the District of Columbia.

Researchers say West Nile may be just one example of an infectious disease whose incidence and geographic range have expanded because of human activities affecting the mosquitoes, birds, rodents and other animals that help spread the infection.

Since the mid-1970's -- a time when it was widely assumed that most infectious diseases had been conquered or at least controlled -- a troubling array of previously unknown diseases has emerged, including Lyme disease, AIDS, mad cow disease, the Ebola virus, Legionnaires' disease and a host of others. In addition, old diseases like yellow fever, malaria and dengue fever have reappeared in their former haunts and spread to new areas. Some microbes, like the ones that cause tuberculosis, malaria and food poisoning, have become dangerously drug resistant.

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In a 2000 report, the World Health Organization identified a half-dozen factors that could affect the distribution and emergence of infectious diseases. The factors include ecological changes like those from global warming and changes in land use; human factors like population growth, migration, war, sexual behavior, intravenous drug use and overcrowding; international travel and commerce; technological and industrial factors like food processing, livestock handling and organ transplants; microbial changes like the development of antibiotic resistance; and breakdowns in public health measures like sanitation, vaccination and insect control.

In the case of West Nile virus, researchers say global warming caused by rising levels of greenhouse gases may be contributing to the warm winters and summer droughts that seem to favor the spread of the virus.

Dr. Paul R. Epstein, associate director of the Center for Health and the Global Environment at the Harvard Medical School, said an important consequence of warming was an increase in ''extreme weather events'' -- droughts punctuated by torrential rains. Drought, he said, helps the mosquito species Culex pipiens, which plays a major role in spreading West Nile.

He added that drought might also wipe out darning needles, dragonflies and amphibians, which destroy mosquitoes. Drought may also aid the spread of infection by drawing thirsty birds to the pools and puddles where mosquitoes breed. ''Hot weather plays a role, too,'' Dr. Epstein said. ''Warmth increases the rate at which pathogens mature inside mosquitoes.''

Climate is not the only factor. As wilderness is developed and animals' specialized habitats are destroyed, opportunistic creatures like rats and crows often take over. Known as generalists or opportunists, animals that thrive near developed areas tend to be hardy species that can eat almost anything and live almost anywhere. If, like crows, they also happen to be capable of carrying a disease and spreading it through mosquitoes to people, they become important factors in outbreaks.

Dr. Epstein described a similar sequence of events for Lyme disease, which is spread to people by ticks that feed on deer and white-footed mice. The factors that helped Lyme disease emerge, he said, include ''the social and human activities that bring us in touch with fragments of forest, like sprawl and suburban life, and the fact that there are lots of deer but few predators of deer.''

In addition, he said, ''warming has contributed to the northern movement of ticks, and warm winters allow for overwintering.''

Globally, warming is widely thought to be contributing to the spread of malaria and dengue, each carried by mosquitoes, to high altitudes in Africa and Central and South America where the diseases had not occurred before.

Slowing emissions of greenhouse gases could help blunt the warming trend in the long run, but the climate system will take decades to respond. And so, many scientists say, it is important to use antibiotics more judiciously, to slow the endless buildup of resistant bacteria. It is also essential, they say, to monitor weather patterns and populations of insects, birds and rodents to anticipate outbreaks and try to head them off. Nations, the scientists add, also have to be on the alert for outbreaks of illnesses with unfamiliar symptoms.

Among recently recognized diseases, one of the most alarming was a brain infection, encephalitis, caused by the Nipah virus, which suddenly appeared at pig farms in Malaysia in 1998 and killed more than 100 people. The outbreak is described in a book, ''Conservation Medicine: Ecological Health in Practice,'' to be published this month by Oxford University Press.

To stop the epidemic, Malaysian authorities killed about a million pigs in 1999. The slaughter dealt a severe blow to pig farming in Malaysia, a $400 million-a-year industry.

Initially, researchers had no idea where the virus was coming from. But in 2000, scientists in Malaysia found that fruit bats were carriers, and suggested that the bats -- perhaps driven out of the rain forest by logging and forest fires that left them hungry -- were attracted to pig farms, where they spread the virus to pigs, which then infected people.

The outbreak is still being studied, and researchers fear that the bats, which migrate, could spread the disease to other countries. Moreover, Nipah can infect dogs, cats and horses, which may also be able to infect people.

In 1997 in Hong Kong, a strain of flu virus jumped from chickens to people, the first time such a virus had gone directly from birds to people without first passing through pigs. More than a million chickens were slaughtered, but researchers did not discover the source of the outbreak or determine what enabled the virus to infect humans.

Human activity is believed to have played an essential role in the birth of the AIDS epidemic. Research has shown that the virus was originally a chimpanzee virus -- simian immunodeficiency virus, or S.I.V. -- and scientists think it jumped species into humans who were exposed to infected blood while hunting and butchering chimpanzees for food or for sale as bush meat.

Now, some researchers say, they fear that a related virus may also make the jump into people. Such a jump could be disastrous, leading to a contagious infection in people -- one that could not be detected by current blood tests.

The scientists' concern stems from an enormous expansion in Africa of the bush-meat trade. It has in part grown because logging roads have opened up remote regions for hunting and shipping of the animal carcasses. Researchers have found surprisingly high rates of S.I.V. infections in meat taken from primates in bush-meat markets in Cameroon. In May, a team from the United States, Cameroon, France and Belgium reported that members had screened more than 700 primate carcasses and found S.I.V. infection in 20 percent of them. More than 30 primate species are now known to carry strains of H.I.V.

Dr. Beatrice Hahn, a professor of medicine at the University of Alabama at Birmingham and a member of the team, said: ''Three things were surprising: the rate of infection, the diversity of viruses and the amount of bush-meat hunting that was going on. It shows for the first time that there is no doubt that humans are routinely exposed to a wide variety of viruses from this activity. We suspect some may have already jumped. And you do not want to transfuse the blood of a person who might have gotten such an infection. You don't have to be freaked out or a doomsday monger, but I think it would be a mistake to ignore it.''

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