Here's a good study on the prevention of the common cold with garlic, and its another illustration of the fact that there is plenty to be found in natural foods that function as efficient medications without the detrimental side-effects. It also shows that the regular inclusion of such foods into your diet can provide good all round protection against acute (short term) and chronic (long term) disease conditions.

There is actually a great natural and very effective all-round remedy that is made using garlic, apple cider vinegar and raw, unprocessed honey that we will detail in a separate article inshaa'Allaah, its a must have in your household.

Josling, Peter, BSc. Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey Advances in Therapy, July 1, 2001, Vol. 18, No. 4, pp. 189-193

One hundred forty-six volunteers were randomized to receive a placebo or an allicin-containing garlic supplement, one capsule daily, over a 12-week period between November and February. They used a five-point scale to assess their health and recorded any common cold infections and symptoms in a daily diary. The active-treatment group had significantly fewer colds than the placebo group (24 vs 65, P < .001). The placebo group, in contrast, recorded significantly more days challenged virally (366 vs 111, P < .05) and a significantly longer duration of symptoms (5.01 vs 1.52 days, P < .001). Consequently, volunteers in the active group were less likely to get a cold and recovered faster if infected. Volunteers taking placebo were much more likely to get more than one cold over the treatment period. An allicin-containing supplement can prevent attack by the common cold virus...

...The common cold is the world's most widespread viral infection, with most people suffering approximately two to five colds per year. Over 200 different viruses cause infection and cold symptoms; the most common, rhinoviruses, account for 30% to 40% of adult colds. Reinfection is prevalent because of this wide variety of infectious viruses. Published literature on the activity of garlic against viral infections is sparse. One report describes that during an influenza epidemic, the former Soviet Union imported more than 500 tones of garlic cloves for acute treatment. Among the viruses sensitive to garlic extracts are the human cytomegalovirus, human rhinovirus type 2, herpes simplex types 1 and 2, and influenza B. Evidence points toward allicin and its condensation product ajoene as the main components in garlic responsible for this antiviral activity. Recently, an allicin-containing supplement (Allimax Liquid and Capsules) has demonstrated efficacy against herpes simplex type 1 and molluscum contagiosum infections...

...This study is the first to use a double-blind, placebo-controlled design to investigate prevention of viral disease with a garlic supplement. The results overwhelmingly favored the supplement as a preventative measure, demonstrating accelerated relief, reduction in the severity of troublesome symptoms such as sneezing, cough and runny nose, and recovery to full fitness. A reduced likelihood of becoming reinfected with other viral strains indicated general improvement in the immune system with the active supplement...

...Of particular note is that volunteers in the active group took the manufacturer's recommended dose of one capsule per day. Over the past 10 years, other published reports on garlic supplements for numerous applications have often used double or triple the actual dose available in retail outlets. The allicin-containing supplement studied may represent a "cure" for the common cold. The results also suggest that infection and reinfection may be effectively prevented by its daily use throughout the year, with an enormous potential saving to national industry in terms of reduced sick days. This product clearly exhibits excellent antiviral activity and warrants further investigation to determine the nature and method of its viral destruction.

You may also want to see a previous article detailing vitamin D deficiency being the "seasonal stimulus" behind influenza here.