What if I told you," Joel Weinstock said, "there were countries where the doctors had never seen hay fever?" It is another piece of evidence, another "aha" moment in the global medical mystery that Weinstock - the chief of gastroenterology and hepatology at Tufts-New England Medical Center - has narrowed down to one chief suspect: the worms. Weinstock, 59, specializes in studying why immunological diseases ... are on the rise in developed countries but remain relatively uncommon in undeveloped countries. He believes these diseases, many of which were almost unheard of 100 years ago, are because of changes in our environment, a lack of exposure to something. And he thinks that something may be the worms. "We realized that one thing people always had was intestinal worms," he said. The parasites that we have been told to avoid - such as hookworm and pinworm - may be the good guys, while excessive hygiene may be the bad guy. "I get about 5,000 e-mails a year from patients all over the world asking what to do," he said. "People know that something isn't right. They keep their kids in the cleanest environments and they get asthma. We get all of these things that were rare becoming common. And a lot of it comes down to hygiene. Excessive hygiene can potentially lead to disease." The "hygiene hypothesis," which was first proposed nearly two decades ago, argues that aspects of cleanliness prevent the immune system from programming itself to fight off disease. "The big question is what are those aspects? We don't want to go back to the standards of the 1800s," Weinstock said. "Public hygiene and cleanliness are very good for us, but removing ourselves entirely from our natural environment is bad for us. We need to figure out the aspects of dirt and exposure that are good for us and hopefully we can find a balance."