Hospitals kill. According to the educational news source Health Sentinel, every year 88,000 people in the United States die from infections acquired in hospitals -- more than the number that die from breast and prostate cancers combined. One reason why this number is so high is that many infections are antibiotic-resistant, meaning they do not respond to one or more of the drugs commonly used to treat them.

Many reasons exist for the prevalence of these impervious germs in hospitals. Hospitals concentrate very sick people into a small area, people whose weakened immune systems leave them highly vulnerable not only to their own illnesses, but also to germs from other patients. Even normally harmless intestinal bacteria can turn deadly in someone with a compromised immune system. Because they treat illness, hospitals dole out large quantities of antibiotics, which encourage resistance not only in their target bacteria, but in other strains as well. These factors contribute to making hospitals what the National Institutes of Health terms "a fertile environment for drug-resistant pathogens."

Other elements also contribute to the development of drug-resistant pathogens in hospitals and elsewhere. Scientists and policymakers are giving increased attention to the large quantities of antibiotics given to farm animals, and their possible contribution to antibiotic resistance in human pathogens. A growing body of circumstantial -- and some direct -- evidence indicates that the two may indeed be linked, and that antibiotics are losing effectiveness as a result.

By definition, antibiotics target bacteria (they have no effect on viruses). Antibiotic resistance occurs naturally in a small percentage of bacteria, which are either able to expel the antibiotic or to find ways around using whatever part of their system the drug targets. Once exposed to an antibiotic, non-resistant bacteria die and resistant ones fill in the population gap, becoming much more prevalent. In evolutionary terms, the antibiotic exerts "selective pressure," giving a competitive edge to resistant bacteria.

Antibiotics are used on farms to treat illnesses in animals, to prevent the spread of disease through herds and to make animals grow more while consuming less food. No one is quite sure why low doses of antibiotics mixed in with food promote growth, but according to a 2001 analysis by the Union of Concerned Scientists, a non-profit environmental group, 25 million pounds of antibiotics are used each year in the United States for growth promotion (the industry group Animal Health Institute puts the number lower, at 3.1 million pounds annually for growth promotion). This compares to 3 million pounds used in human medicine.

In 1998, a 62-year-old Danish woman died when the food poisoning she contracted from eating Salmonella-infected pork failed to respond to the antibiotic ciprofloxacin. Researchers led by Henrik Wegener of the Danish Veterinary Laboratory were able to genetically match the drug-resistant Salmonella strain to one in a specific herd of pigs. Though the pigs had not been treated with ciprofloxacin, nearby herds were treated with a similar drug, and resistant bacteria had moved between farms.

Demonstrating a direct link between antibiotic use on the farm and resistant, disease-causing bacteria was an important step for the researchers. "It's the closest that anyone has come to a smoking gun," says Abigail Salyers, a microbiologist at the University of Illinois, Urbana-Champaign.

One striking aspect of this story is the belief Wegener has that his country has the most aggressive surveillance system for resistant Salmonella in the world, meaning that if it had happened somewhere else, the source of the ciprofloxacin-resistant germs might never have been identified.

Another case, reported in 2000 by Paul Fey of the University of Nebraska medical center, was that of a Nebraskan boy who contracted the same strain of Salmonella that infected some cows on the ranch where he lived. The cows had been infected by cattle on a nearby ranch that had been treated with the antibiotic ceftiofur. When the boy's doctors treated his infection with a similar drug, ceftriaxone, the Salmonella turned out to be resistant. Fortunately, the boy recovered anyway.

Although cases in which such a direct link has been identified are rare, disturbing trends can be seen in larger-scale studies. For example, in 1994 the Food and Drug Administration (FDA) approved the use of a class of antibiotics known as quinolines, which are also used in medicine, for prevention of infection in chickens. In the next seven years the percentage of people with quinoline-resistant Campylobacter, an intestinal bacteria, rose from 1% to 17%, according to the Minnesota Department of Health.

Until 1997, the glycopeptide avaroparcin was used as a growth promoter in European livestock. Even in parts of Germany where another glycopeptide, vancomycin, is rarely prescribed, vancomycin-resistant Enterococci (VRE) - another intestinal bacteria - was fairly common in people. In 1997 the European Union banned the use of avaroparcin as a growth promoter, and between 1996 and 2001 researchers at the University of Antwerp saw VRE in Belgian hospitals drop from a prevalence of 5.7% to just 0.6%.