Healthy animals are becoming a major threat to sick humans — especially to the growing number of people infected with bacteria resistant to commonly used antibiotics.

Humans usually get powerful infection-fighting drugs only when they are ill. But chickens, pigs and cows raised in factory farms routinely are given antibiotics, the same kinds used in people. The drugs are given to help them gain weight for market and avoid the diseases that thrive in close living quarters, where animals are packed in cheek-by-jowl. The result is a slow but steady increase in drug-resistant bacteria that affects not only humans but sick animals as well.

A bill introduced this month in the U.S. Senate would rein in the indiscriminate use of antibiotics in animal feed. The Preservation of Antibiotics for Medical Treatment Act would require farmers to stop routinely giving antibiotics to healthy animals. It would apply both to drugs used in humans and those considered crucial for treating sick animals, for whom the effectiveness also is being eroded.

The bill, sponsored by Sens. Edward Kennedy, D-Mass., Olympia Snowe, R-Maine, Sherrod Brown, D-Ohio, and Jack Reed, D-R.I., also would set aside money to help farmers with costs of making the switch.

As recently as two decades ago, some scientists were predicting a rosy, not-too-distant future in which infectious disease would be tamed by rapid advances in medicine, sanitation and vaccines. What has come about instead is the emergence of new species-jumping infectious diseases such as AIDS, SARS and Hantavirus, and the re-emergence of deadlier forms of older diseases, such as multi-drug-resistant tuberculosis.

The development of new antibiotics has slowed in recent years. Meanwhile, hospitals and nursing homes increasingly have been faced with infections by micro-organisms that are resistant to even the most powerful drugs.

Staph infections caused by bacteria that is resistant to common antibiotics, once found only among the elderly and ill, now routinely occur among high school athletes.

The Institute of Medicine, part of the National Academies of Science, has warned for years that overuse of antibiotics contributes to those problems. In 2001, Congress passed legislation to discourage doctors from inappropriately prescribing the drugs.

But unless widespread use of the antibiotics by farmers is curbed, that good work will continue to be undermined. The Senate bill is an important first step toward accomplishing that.