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Drinking alcohol

You may want to reconsider that night on the town during the height of flu season: You could wake up the next day with something much worse than a hangover. A study in BMC Immunology found that mice who consume large quantities of alcohol in a short period of time are left with weakened immune systems and might have a harder time fighting off infections for at least 24 hours.

Another side effect from drinking too much: Alcohol can quickly and easily dehydrate you, which interferes with your nose's and throat's ability to trap germs and expel them in the form of mucus.

Next: Relying solely on antibacterial hand gel