This is from the book The Healing Power of Herbs by Michael Murray:
Antimicrobial activity
Garlic exerts broad-spectrum antimicrobial activity against many genera of bacteria, virus, worms, and fungi, as summarized in several works.4-6 These findings seem to support the historical use of garlic in the treatment of a variety of infectious conditions.
Antibacterial activity
As far back as 1944, studies have demonstrated that both garlic juice and allicin inhibited the growth of Staphylococcus, Streptococcus, Bacillus, Brucella, and Vibrio species at low concentrations.7-8 In more recent studies using serial dilution and filter paper disk techniques, fresh and vacuum-dried powdered garlic preparations were found to be effective antibiotic agents against many bacteria, including Staphylococcus aureus, alpha- and beta-hemolytic Streptococcus, Escherichia coli, Proteus vulgaris, Salmonella enteritidis, Citrobacter species, Klebsiella pneumoniae, and mycobacteria.4-6,9,10 These studies compared the antimicrobial effects of garlic with those of commonly used antibiotics, including penicillin, streptomycin, chloramphenicol, erythro-mycin, and tetracyclines. Besides confirming garlic's well-known antibacter-ial effects, the studies demonstrated this herb's efficacy in inhibiting the growth of some bacteria that had become resistant to one or more of the antibiotics.
Antifungal activity
Garlic has demonstrated significant antifungal activity in many in vitro and in vivo studies.4 11-16 From a clinical perspective, its inhibition of Candida albicans is most significant¡Xboth animal and test tube (in vitro) studies have shown garlic to be more potent than nystatin, gentian violet, and six other reputed antifungal agents.4,12-14
I offer the following as an illustration of just how powerful garlic's anti-fungal action can be. In one study at a major Chinese hospital, garlic therapy alone was used effectively in the treatment of cryptococcal meningitis, one of the most serious fungal infections imaginable.15
Antithelmintic effects
Garlic extracts exert anthelmintic activity against common intestinal parasites including Ascaris lumbricoides (roundworm) and hookworm.17,18
Antiviral effects
Garlic's antiviral effects have been demonstrated by its protection of mice from infection with intranasally inoculated influenza virus, and by its enhancement of neutralizing antibody production when given with influ- enza vaccine.19
In vitro, fresh garlic, allicin, and other sulfur components of garlic killed herpes simplex types 1 and 2, parainfluenza virus type 3, vaccinia virus, vesicular stomatitis virus, and human rhinovirus type 2. 20 The order of viru cidal activity was as follows: ajoene > allicin > allyl methyl thiosulfinate > methyl allyl thiosulfinate. Ajoene is found in oil-macerates of garlic/but not in fresh garlic extracts. No activity was detected when alliin, deoxy-alliin, diallyl disulfide, and diallyl trisulfide were tested. Fresh garlic extract was virucidal against all viruses tested; but the virucidal activity of commercial products depended how they were prepared. Those products producing the highest level of allicin and other thiosulfinates had the best virucidal activity.20