This relative of the onion, shallot and leek has been around a very long time. The Old Testament book of Numbers mentions the Israelites eating garlic, and we know it was widespread in the Roman Empire. Having a long history, there is a lot of garlic folklore from its ability to fend off vampires, to it being a cure for cholesterol. Of course only some of the medicinal claims of garlic can be tested.
When subjected to a randomised clinical trial, garlic's reputed ability to lower cholesterol was completely absent (1). However, the ability of garlic to protect against the effect of cholesterol on arteries, and even reduce the accumulation of plaque has mounting evidence. Garlic is also reputed to protect against and cure the common cold, have anti bacterial properties and be a powerful anti fungal.
Garlic is a good source of Vitamin C, which we know is an excellent anti-oxidant, which is hugely beneficial for our bodies. Garlic also contains the following in varying amounts: allicin, beta-carotene, beta-sitosterol, diallyl disulfide, oleanicolic Acid, phytic acid, Calcium, folate, iron, magnesium, manganese, potassium, selenium, zinc and vitamin B6 to name a few - an impressive list. Garlic's rich mix of compounds includes those known to help prevent inflammation (through inhibition of lipoxygenase and cyclooxygenase), which can combine with the vitamin C and go some way to explaining the powerful medicinal reputation of garlic.
Garlic also contains, allicin, which is not only responsible for its powerful and distinctive smell, but its action as an anti-bacterial and anti fungal. In lab tests, raw garlic kills bacteria and fungus very effectively indeed. Also the presence of diallyl sulphide has raised hopes that garlic has anti-carcinogenic properties because diallyl sulphide inhibits the conversion of the heterocyclic amine PhIP into DNA damaging compounds.
Garlic has a proven ability as an antifungal and anti-bacterial from herbal medicine through the centuries. Evidence is mounting that it is good at protecting our arteries from formation of plaques, and it has all the compounds necessary to be an effective anti-inflammatory while containing an excellent nutrient profile in its own right. However, the ability of garlic to ward of vampires and prevent cancer needs a lot more investigation!
So we know garlic is excellent - now for the bad news, cooking degrades the active ingredients, completely destroying allicin, and significantly degrading diallyl sulphide and of course damaging vitamin C. The health benefits refer in the most part to crushed raw garlic and garlic extract. In addition odourless varieties have been shown not to share the benefits of their smelly brothers.
Finally - why do we smell of garlic when we eat it? This is because garlic's powerfully smelly sulphur compounds are metabolised to form ally methyl sulphide which cannot be digested and are excreted via the lungs and skin. Parsley does indeed mask the smell - but its effects are short lived. If you want rid of the smell - take a sauna - on your own!
Reference
Effect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a randomized clinical trial. Gardner, C. et al. Arch Internal Medicine. 2007 Feb 26; 167(4):346-53.