Supplements of an aged garlic extract may reduce the days that a person suffers from cold and flu. The supplement was also found to enhance immune cells, particularly natural killer cells and gamma delta cells. Garlic also contains compounds with potential to modulate the immune system such as lectins, fructooligosaccharide, and apigenin. Researchers recruited 120 healthy subjects and randomly assigned them to receive a daily supplement of the aged garlic extract (2.5 grams) or placebo for 90 days. Results showed that the number of both types of immune cells increased more in the garlic group compared with the placebo group. Significant improvements were also reported for the number of symptoms, the number of days and incidences that people felt under the weather, as well as a decrease in days missed at work.

See http://www.raysahelian.com/garlic.html. Fresh garlic works better but the downside to fresh garlic is that the breath odor can stay as long as 2 days, so it is not appropriate to those who are in close contact with others.