According to him, recent researches have shown that society's anti-bacterial fight may contribute to asthma and allergies, and perhaps even mental illnesses.
"Microorganisms shape the lives of all living things and infections steer the course of the world. Most people understand that infections are at the root of many terrible diseases like malaria and leprosy," he said.
"But infection may also play a significant role in many chronic aliments, including some that may surprise you such as schizophrenia, ulcers and obsessive compulsive disorder," he added.
The role Toxoplasm gondii, a bacterium, may play in producing schizophrenia in people isn't clear. But rats infected with T. gondii exhibit suicidal behaviours. And otherwise normal men and women infected with T. gondii exhibit alterations in their behaviours: Infected women are likely to be more warm-hearted and outgoing, and infected men often are more jealous and suspicious.
About 15 percent of all cancers could be prevented if infectious diseases that play a role in causing these cancers, which include stomach, cervical and liver cancers, could be controlled, Callahan said.
But what many people may not realize is that most infections ensure our health instead of compromise it, he said.
Mitochondria are bacteria that take fats and sugars and make adenosine triphosphate, or ATP. Every action that distinguishes a living human being from a dead human being is dependent on ATP.
"We need our bacteria," Callahan said.
Other research shows that sheltering a child from bacterial infections increases his or her chances of developing asthma and allergies. In fact, recent studies show that the more educated parents are, the more likely their children are to develop asthma and allergies possibly because these parents are more likely to worry about bacterial infections.
"We understand that part of becoming an adult is learning to interact with people and recognize both bad and good in those people. The same is true for bacteria and other infectious microorganisms," he said.
Humans have 10 times more bacterial cells in their bodies than human cells. Without bacteria, there would not be humans. Human life depends on certain infections.
"Before we knew the important role that infections play, we knew about things like rabies and polio and yellow fever. Because of that, when Fleming finally introduced penicillin in 1945, we went crazy with joy and began to slather everything with antibiotics, especially ourselves. Now we are paying our dues for that overreaction," he said.
Callahan points out that there are more bacteria by far in this world than any other living thing.
"We are a minority on this planet, and we must learn how to work with the majority," he said.