I have found that in most cases where I notice my throat is getting sore, or my nasal passages are raw, indicating a cold is coming on, if I eat a clove of garlic, the symptoms abate within the day and never develop into a full-blown cold.
Most recently I tried it at the onset of a flu (characterized by rapid onset of high fever without respiratory symptoms). The garlic failed to abate the symptoms. Instead, I had purgatory runs, and was flat on my back the next day. However, after drinking one ounce of very old Noni juice from my refrigerator, I felt almost back to normal the next day. There is no way to know, of course, what I would have been like without the Noni.
Now, that's just my personal experience. But there is a ton of scientific research on garlic. For example,
This study was designed to investigate the effect of heating, storage, and ultraviolet exposure on antimicrobial activity of garlic juice and its bacteriocidal activity against common human pathogens. Antimicrobial activity of fresh garlic juice was tested against Escherichia coli, Staphylococcus aureus, Streptococcus hemolyticus B, S. hemolyticus A, Klebsiella sp., Shigella dysenteriae, and Candida albicans using the disc method. The dilution method was performed by addition of garlic juice to broth media to obtain 1-100% concentrations as vol/vol or wt/vol. Garlic juice was used after 24 hours of storage at 4 degrees C, heating to 100 degrees C for 5 minutes, 10 minutes, 30 minutes, and 60 minutes, heating to 80 degrees C for 60 minutes, and 4 hours of exposure to ultraviolet light. Re-culture of specimens taken from garlic-induced negative media was performed in fresh broth free of garlic juice. Results showed that all the isolates were sensitive to fresh garlic juice; the most sensitive was C. albicans, and the least sensitive was S. hemolyticus A. Heating to 100 degrees C for 30 and 60 minutes completely abolished the antimicrobial activity, while heating for 5 and 10 minutes, storage for 24 hours, and 4 hours of ultraviolet exposure decreased it. Garlic juice was bactericidal at concentrations of 5% and more. Thus garlic juice has marked antimicrobial activity that makes it a potential agent to be tested in clinical trials. The antimicrobial activity was compromised by storage and heating; therefore it is advisable to use fresh garlic and avoid boiling it for more than 5 minutes during cooking.
Abstract: Garlic is a popular spice added to several edible preparations and is a remedy for a variety of ailments. Epidemeological as well as laboratory studies have shown that garlic consumption reduces certain cancer incidences in the stomach, colon, mammary, cervical, etc. This article focuses on the general chemistry, metabolism, anticarcinogenic properties, mechanism of action behind the anticarcinogenic effects, functional foods based on garlic; and future areas of research. Garlic has been shown to metabolized into N-aceryl-S-allyl cysteine, allyl mercaptan, diallyl disulfide, diallyl sulfide, diallyl sulfoxide, diallyl sulfone, and allyl methyl sulfide. Garlic has been thought to bring about its anticarcinogenic effect through a number of mechanisms, such as the scavenging of radicals, increasing gluathione levels, increasing the activities of enzymes such as glutathione S-transferase, catalase, inhibition of cytochrome p4502E1, DNA repair mechanisms, prevention of chromosomal damage etc. Future research should standardize the dosage of garlic and type, ie., whether it should be taken fresh, cooked, or aged. The formulation of odorless functional foods with the retention of anticarcinogenic activity should be further studied.
Abstract: Evidence that garlic inhibits platelet aggregation, increases fibrinolysis, reduces blood pressure, enhances anti-oxidant activity, and reduces serum lipids suggests that it may have cardio-protective properties. The lack of qualitative standardization of garlic preparations and the methodological weaknesses of earlier studies makes comparison between different studies complicated. Quantitative pooling of data in meta-analyses of the primary trials strongly suggests that garlic is an effective lipid-lowering agent.Go to PubMed and search on "garlic" to find more.