Cat dander has become a complete puzzle, with some studies finding that exposure early in life leads to asthma, and others saying it protects against asthma. At this point, nobody knows which study to believe, but most experts agree that when people already have asthma, being around cats can make it worse.

Under some of the theories, I should have had the world’s worst asthma. Clearly, I had allergic tendencies, and the experts would have shuddered at my environment. I grew up in New York in a small apartment with parents who were heavy smokers, on a busy street with trucks rumbling by and a bus stop in front of the door. Buildings all around us burned coal. At times, we had a dog, a cat, parakeets and, briefly, a duck. It’s amazing that the pets survived the smoke. My mother was a decent housekeeper, but she wouldn’t have won any prizes. I never wheezed. Go figure.

Obesity and asthma have also been linked in some studies, but the link, if it exists, is not understood. Researchers say it is simply not a matter of asthmatic children growing fat because they cannot exercise. The weight gain can be first.

Nutrition is another mystery. Studies of fruits, vegetables, cereals, fatty acids, vitamins, minerals and antioxidants have been inconclusive, and little is known about the effects from what pregnant women eat. Experiments in which pregnant women avoided cow’s milk and eggs in hopes of preventing asthma in their infants did not work, and breast-feeding doesn’t prevent the disease, either.

One theory that has received attention recently is the “hygiene hypothesis,” the idea that children today are raised in homes that are too clean and that asthma is somehow caused by the lack of exposure to infections and bits of microbes early in life. Under this theory, germs are supposed to help the immune system develop normally, and without them the system may overreact to other substances in the environment, producing allergies and asthma.

There is some evidence to support the idea. Studies find that children raised on farms are less prone than others to asthma, maybe because they are exposed to plenty of microbes in barns and stables. But the connection is still not fully understood, and some viral infections clearly make asthma worse.

A related idea is that the increased use of antibiotics in recent decades contributes to asthma by changing the type of the bacteria that live in the gut. But that has not been proved. Some researchers have suggested that acetaminophen, used to treat pain and fever, may be linked to asthma. Its use increased in the 1980s, after pediatricians declared aspirin unsafe for children. But that theory has not been proved, either.