hen Kathy T. Nguyen died of inhalation anthrax in New York on Oct. 31, the police and medical investigators were quickly deployed to find the source of the spores that had infected her. They interviewed 232 co-workers, 27 neighbors and 35 acquaintances in an effort to reconstruct her final two months. They searched her apartment and swabbed surfaces there and in her workplace and the subway stations she used. They vacuumed her clothes in search of spores. They used her subway fare card to trace her path around the city, studied her phone records and inspected her usual laundry, post office and grocery store.

They never found a single spore or any other clue to how Ms. Nguyen became infected.

Describing the investigation at a medical meeting last week in Atlanta, Dr. Timothy Holtz, a preventive medicine fellow at the New York City Health Department, concluded with a slide that said, "We will likely never know."

The Atlanta meeting, attended by 2,000 doctors and scientists, was an international conference on emerging infectious diseases, organized by the Centers for Disease Control and Prevention, the World Health Organization and other groups. "Emerging" refers to newly discovered infectious diseases or old ones that have rebounded, turned up in new places or become drug resistant, and whose incidence has increased in the last two decades or threatens to rise soon. Previous conferences focused on malaria, AIDS, tuberculosis, yellow fever, mad cow disease and West Nile encephalitis. Now, anthrax and other potential bioterror agents — smallpox, plague, tularemia, botulism, Q fever and the Ebola and Marburg viruses — have been added to the ranks of emerging infectious diseases.

Addressing the conference, Dr. James Hughes, the director of the C.D.C.'s National Center for Infectious Diseases, said that although the anthrax attack last autumn was small — it killed 5 people and made 17 others ill — it quickly overwhelmed the nation's laboratories.

"We learned we were not adequately prepared," he said, noting that America's capacity to deal with a sudden increase in need for services was deficient not only in laboratories, but also in the production of vaccines and antibiotics and the availability of hospital beds. He said the ability to detect outbreaks promptly also needed improving. Dr. Hughes added that bioterrorism and naturally occurring outbreaks of infectious disease would "complicate U.S. and global security over the next 20 years."

The federal government plans to distribute $918 million to the states next month for bioterrorism preparedness. Dr. Hughes and other experts say this is the time to improve the entire public health system to combat other threats from infectious disease, like flu epidemics, antibiotic resistant bacteria and illnesses spread by food, water, insects and animals. In the United States, for instance, 76 million people a year get sick from food-borne infections, and 1 in 1,000 are hospitalized. Over all, the cost is $6.5 billion.

Diseases carried by mosquitoes and ticks are also a considerable problem in the United States. Dr. Duane Gubler, a specialist in insect-borne diseases who works for the C.D.C. in Fort Collins, Colo., said West Nile encephalitis, never detected in the United States before it occurred in New York in 1999, had spread faster than expected.

In 2001, people were infected in nine states and birds in 27 states. Although the disease may cause only a mild illness in healthy people, it can be severe and even fatal in older people. Of 149 known cases, 18 people have died; but there may be more cases that have not been reported because victims did not get very sick. The virus is carried by 29 species of mosquitoes, 100 species of birds and numerous mammals.

"It will continue to spread and will be a major public health problem in the next decade," Dr. Gubler said. "Most states should consider themselves at risk."

He said dengue fever, another viral disease carried by mosquitoes, had become a major problem worldwide, with 50 million to 100 million cases a year, including 200,000 to 500,000 taking a hemorrhagic form that can be fatal. Before 1980, he said, dengue was not a problem in Central or South America, but now it is endemic in 27 countries, including Brazil, which Dr. Gubler described as in the throes of a "roaring epidemic." Last year Hawaii had its first dengue outbreak in 56 years, on Maui.

"My guess is that the next global public health emergency will be yellow fever," Dr. Gubler said.

Yellow fever is also carried by mosquitoes, and epidemiologists expect epidemics to occur in cities in South America, and also Asia and the South Pacific. "It will get here by plane," Dr. Gubler said, "but I don't anticipate a major epidemic here."

A vaccine exists, but it is in short supply.

The comeback of insect-borne diseases has revealed a glaring lack of trained people who know how to collect and identify mosquitoes, and who know how insects interact with microbes and people to spread disease. The field, known as vector biology or medical entomology, began shrinking in the 1970's because diseases carried by insects had diminished so much that there were few jobs for vector biologists.

When West Nile encephalitis broke out in New York, Dr. Gubler said, the city had trouble finding a medical entomologist. Dr. Gubler said he and other researchers hoped that the C.D.C. and the National Institutes of Health would develop training programs to help bring the field back.

Another concern is the spread of antibiotic resistant bacteria, which cause infections that can be very difficult to treat. Dr. Keith Klugman, a professor of infectious diseases at Emory University, described a "global pandemic" of antibiotic resistance in the bacterium Streptococcus pneumoniae, commonly called pneumococcus. It is a common cause of respiratory illnesses and kills more than three million children a year.

The pneumococcus also causes ear infections, and many studies show that the more a child is treated with antibiotics, the more likely resistance is to develop. Low doses of antibiotics given for a long time are especially likely to breed resistance. Particularly worrisome is that the bacteria are becoming resistant to fluoroquinolones, a powerful and much-needed class of drugs. Dr. Klugman called fluoroquinolone resistance a "potential disaster" and said he expected it to increase greatly in the United States in the next four to five years.

Possible solutions include vaccinating children to prevent pneumococcal infections and developing better diagnostic tests to tell viral respiratory infections from bacterial ones, so that antibiotics are not given unnecessarily. Research is also needed, Dr. Klugman said, to determine the best way to use antibiotics; in some cases, the widely prescribed 10-day regimens may be contributing to resistance, and shorter courses with higher doses may work just as well and cause fewer problems.

Speakers at the Atlanta conference also warned that people can catch common infections in utterly unexpected ways.

When salmonella, a common food poisoning infection, broke out last spring at an elementary school in Minnesota, state and county health departments first investigated the school lunch program. But they noticed that the children most likely to be infected belonged to the science club or to another after-school program, the adventure club. Interviews revealed that the science club had been dissecting owl pellets, clumps of indigestible bone and fur that owls regurgitate after eating.

In Minnesota, the pellets were being dissected on a table in the cafeteria. After the science club was finished, the adventure club ate its snacks off the same table, without its being washed. In fact, the table was not washed until the following day — by which time another group of children had eaten off it.

The source of the pellets was a barred owl at a local nature center. Cultures of leftover pellets and the owl's droppings turned up the same strain of salmonella found in the children; the bacteria almost certainly came from the owl's diet of thawed, uncooked chicks.

Fred Anderson, an epidemiologist in Washington County, Minn., said there was no reason to ban owl pellets from school programs. But, he suggested, the pellets should not be dissected in the lunchroom. And the young scientists must wash their hands when they are finished. Better still, owl pellets can be sterilized in an oven, or sterilized pellets can be bought from commercial suppliers.