In a subgroup of six studies, 642 people were exposed to severely stressful arctic cold or intense physical activity. Those who took 200 mg to 2,000 mg of vitamin C per day had half as many colds as placebo.
Doctors believe vitamin C stimulates infection-fighting white blood cells.
In lab studies, echinacea activated white blood cells that produced antibodies to fight bacteria and viruses.
Echinacea purpurea is the most common form of echinacea.
In four of six echinacea purpurea studies, 764 healthy adults with cold symptoms who took echinacea saw milder symptoms compared to placebo, and cold durations shortened by 1.5 to 4 days.
Researchers said allicin, the active agent in garlic that causes its odor, contains antiviral properties.
In one study of 146 healthy adults during 12 weeks of winter, those who took an allicin extract for 12 weeks had 64 percent fewer colds compared to placebo.
Zinc is an essential mineral which, when deficient, raises chances for infection.
In four of five studies using 13 to 23 mg of zinc lozenges every two hours, participants saw milder symptoms and cold durations shorten by 1.3 to 6.9 days.
In 13 zinc lozenge studies, while there was no effect in five of the studies that used less than 75 mg of zinc per day, in five studies that used more than 75 mg of zinc per day, colds shortened by 20 percent.