While most of us hack, wheeze, and sneeze our way through winter, there’s always one person we know who somehow never gets sick. What is he doing that the rest of us aren’t? We decided to find out, diving into the latest research on influenza and colds, as well as fact-checking conventional wisdom and old wives’ tales. And just in time: December, January, and February are the worst months for colds and the flu — thanks to dry, chilly air (conditions in which viruses are most contagious) and the fact that we spend our days crowded indoors among co-workers, fellow commuters, and family members, at least one of whom should probably be home in bed. Each season, as many as 20 percent of us will come down with the flu, and odds are that most of us will get at least one cold. You could beat those numbers if you follow this advice. And if you do get sick — or if you already are — our expert tips will help you feel better, faster.