Infectious disease experts say there are a number of factors fueling this hot, sneezy summer. While pandemic lockdowns protected many people from Covid-19, our immune systems missed the daily workout of being exposed to a multitude of microbes back when we commuted on subways, spent time at the office, gathered with friends and sent children to day care and school.

Although your immune system is likely as strong as it always was, if it hasn’t been alerted to a microbial intruder in a while, it may take a bit longer to get revved up when challenged by a pathogen again, experts say. And while some viral exposures in our past have conferred lasting immunity, other illnesses may have given us only transient immunity that waned as we were isolating at home.

“Frequent exposure to various pathogens primes or jazzes up the immune system to be ready to respond to that pathogen,” said Dr. Paul Skolnik, an immunovirologist and chair of internal medicine at the Virginia Tech Carilion School of Medicine. “If you’ve not had those exposures, your immune system may be a little slower to respond or doesn’t respond as fully, leading to greater susceptibility to some respiratory infections and sometimes longer or more protracted symptoms.”