As per Medical Herbalism, "The major odor principle of garlic, allicin is produced from the amino acid alliin by the enzyme alliinase when exposed to air. In both laboratory and clinical studies, allicin has demonstrated antidiabetic, antihypertensive, antibiotic, and hypolipidemic activities. It also enhances fibrinolytic activity in the blood and inhibits platelet aggregation." 
Garlic also contains B vitamins, minerals and flavonoids.
This means that therapeutic doses of garlic will indeed have an antimicrobial effect against certain bacteria, viruses, fungi and parasites. What's more, the volatile oil of the garlic gets excreted through the lungs. This makes it especially useful to use for upper respiratory tract infections.
For the cardiovascular system, garlic can be considered a healthy tonic that is preventative against chronic heart diseases. Not only does it lower blood pressure and blood sugar, but it promotes the production of HDL, the good cholesterol, and inhibits LDL, the dangerous lipoprotein. This prevents atherosclerotic build up in the arteries. It is a blood thinner because it lowers the activity of platelet activating factor (PAF) in addition to enhancing the break down of blood clots, which prevents sudden occlusion of arteries and veins.
Hyperlipidemia: There is contradictory evidence about the effects of garlic on cholesterol and triglyceride levels. Initial research in the early 1990s, mostly using a specific garlic powder (Kwai, Lichtwer Pharma) 300 mg three times daily for 12-24 weeks, showed that garlic modestly reduces total cholesterol, low-density lipoprotein (LDL) cholesterol, and triglyceride levels (279,4782,4783,4784,4785,4786,4787,4788). Some evidence also showed that taking very high doses of a specific aged-garlic extract (Kyolic, Wakanuga) 7.2 grams daily for 24 weeks modestly reduces LDL cholesterol (1873,1875). However, many of these trials have serious design flaws and are low-quality studies (4786,4788).
Later research has more consistently shown several garlic preparations to be ineffective, including the previously studied specific garlic powder product (Kwai, Lichtwer Pharma) (731,4792,4793,4795,4807), another specific garlic powder product (Garlicin, Nature's Way), an aged garlic extract product (Kyolic-100, Wakanuga), raw garlic (15295), a garlic oil product (Tegra, Hermes) (732), and other garlic preparations (4794,4795,15296).
One analysis of garlic studies suggests that garlic might have short-term benefits on lipid levels after 1-3 months of treatment, but no significant benefit after 6 months (6897). Another analysis shows that when the results of all garlic studies are pooled, there appears to be some benefit; however, when only higher-quality studies are analyzed there is no significant benefit (6457).
Advise patients with hyperlipidemia that taking garlic supplements is unlikely to provide a clinically significant reduction in cholesterol or triglyceride levels. 
The flavonoids are potent antioxidants and therefore protect our bodies against oxidative stress; especially the oxidation of lipids, which when not prevented can lead to atherosclerotic build up in the vasculature as well. In addition, thanks to these antioxidants, "according to epidemiological evidence, garlic may have cancer-preventative properties, especially against cancers of the gastrointestinal tract." 
Regarding Blood Pressure:
Hypertension: Some clinical research shows that taking garlic orally can modestly reduce blood pressure in patients with hypertension and in people with normal blood pressure (277,278,279,1873,6897,16605). In one analysis, garlic reduced systolic blood pressure by about 8% and diastolic blood pressure by about 7%, compared to placebo in patients with hypertension (16605). Most studies of garlic for hypertension have used a specific garlic powder formulation (Kwai, Lichtwer Pharma); however, an aged garlic extract has also been used (1873). 
[1, 2] Hoffmann, David. Medical Herbalism: The Science and Practice of Herbal Medicine. Rochester, VT: Healing Arts, 2003. Print.