It's been discovered that ulcers can be the result of viral infections, against which garlic can be a useful, internally taken, substance. It also is helpful against parasites, bacterial infection, cancer, and even slows the growth of existing tumors. Garlic works inside the body by promoting the ability of white blood cells to fight infections, and by stimulating other immune cells like macrophages and T-cells which fight viral and bacterial infections, as well as cancer cells. In other words, it helps your body to help itself, and neither bacteria nor virus can develop a resistance to that. It's still under debate.
Some studies say one thing, some say another.
Standardized doses make it easier to track the effectiveness, but there are questions as to the effectiveness of garlic at all after it's been tampered with. The reasoning is that the power as well as the taste and odor of garlic is released when a clove is bruised, cut or chewed. Allicin thus released oxidizes quickly and breaks down into over a hundred different substances, not all of which are proved to be helpful to the human body. It's best to go with what we know to be true. Raw garlic is an antibiotic, taken internally or externally.