Editor’s Note: This page originally discussed early research exploring a possible connection between childhood vaccination and asthma. Since that article was written, many additional studies have examined this question. The original article has been retired because it no longer reflects the current body of evidence.
Researchers continue to study why asthma has become more common over the past several decades. Proposed explanations include changes in air pollution, allergies, obesity, vitamin D status, alterations in the gut microbiome, reduced exposure to infections early in life (the “hygiene hypothesis”), and other environmental and genetic factors. Asthma is a complex disease, and no single factor has been shown to explain the increase in cases.
