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Why Do People Get Sick When the Seasons Change?

Rather, the temperature shifts permit a different group of viruses to flourish, and it's these viruses that make people sick, said Dr. Benjamin Kaplan, an internal-medicine physician at Orlando Health in Florida.

"Many studies show that rhinovirus and coronavirus are the two main agents of the common cold," Kaplan told Live Science. "Interestingly, they flourish in cooler weather, such as what we have in spring and fall." [Is It Safe to Hold in a Sneeze?]

Likewise, the influenza virus replicates and spreads most effectively when the air is cold and dry; "hence, people [tend to] get the flu in the wintertime," he said.

Summer illnesses arise from a combination of several factors. People with seasonal allergies often feel congested and develop runny noses and itchy eyes when they're near pollen, mold or grass. Their immune systems may go into overdrive as they react to these allergies, leaving them more vulnerable to viral contagions, Kaplan said.

However, these summer infections are usually milder than those caught in other seasons, he said.

Sometimes, people may mistake their allergies for a cold, Kaplan added. "In fact, it is said that the term 'hay fever' actually comes from the time when farmers would be collecting the hay at the end of summer and became 'sick with fever,'" he said. "But really, they were just suffering from severe exposure to allergens from the fields." [Sniffle Detective: 5 Ways to Tell Colds from Allergies]

To avoid getting sick, it's best to follow this commonsense advice: "Practice good hand washing, get plenty of exercise, eat healthy and make sure to get at least 6 to 8 hours of restorative sleep [a night]," Kaplan said.

Also, don't be hoodwinked by flashy cures. Over-the-counter supplements— including Echinacea, vitamin C and zinc — are not significantly effective in preventing or curing colds, according to scientific studies, Kaplan said.

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