Irregular Webcomic! #4855

Of all the signs that someone might be a vampire, I feel like the not liking garlic thing was a bit of an afterthought.

I did a bit of research into the origins of this particular bit of vampire lore, and the purported reasons for why vampires dislike garlic, only to discover that everybody has theories, but nobody seems to actually know. Theories that I found include:

Quite simply: garlic stinks. They just don't like it.
Anything stinky repels vampires, because they have a heightened sense of smell.
Specifically, garlic is a "clean" stinky odour. It's a foodstuff that people can eat safely, as opposed to other stinky things like dung that are "dirty".
The strong smell of garlic helped to dissipate "bad air" that could cause diseases. By extension, it could also ward off evil.
Vampires are suave and seductive. it would be difficult for them to seduce anyone with garlic breath, thus they avoid it.
Garlic is known to repel mosquitoes, so by extension it also repels other bloodsuckers such as vampires (although it doesn't repel all bloodsucking creatures - leeches are actually attracted to garlic).
Garlic contains a compound called allicin which has antibiotic properties. Some beliefs liken vampirism to an infectious disease, and the idea is that an antibiotic will kill the infectious agent. (It's not clear when these beliefs arose, or if they predated either the germ theory of disease or knowledge of antibiotics.)
Garlic contains a compound called ajoene which has anticoagulant properties. Vampires must also have some anticoagulant properties to enable them to drink blood without it clotting, so additional anticoagulation puts them at risk of haemorrhages, thus they shun garlic.
According to some folklorists, the vampire myth may stem from people suffering from porphyria, which can make your skin pale and your gums shrink, thus emphasising the size of your teeth. Garlic can apparently make these symptoms worse, so people suffering porphyria were advised to avoid it.
A different porphyria-related explanation is that sufferers of the disease have an intolerance for foods with a high sulphur content, of which garlic is one.
According to some folklorists, the vampire myth may stem from people suffering from rabies, which can lead humans to foam at the mouth, attempt to bite people, adopt a nocturnal activity cycle, and become hypersensitive to sunlight... and the smell of garlic.
Evil spirits of all types, from multiple traditions around the world, apparently hate garlic. So of course vampires, being evil supernatural beings, do to. Although this just kicks the can down the road a bit by not explaining why evil spirits hate garlic.
Garlic can cause a spiritual awakening. In some traditions garlic is said to detoxify the body and decalcify the pineal gland, or "third eye", leading to increased spiritual awareness. Vampires, being spiritually nasty beings, are afraid of this.
Garlic lowers blood pressure. So... vampires would prefer someone with a nice high blood pressure to feed from. (Yeah, this sounds weird to me too. I mean, weirder than some of these others.)
Garlic represents the humble life of the poor. It is cheap and common and used for flavouring food. To a vampire, garlic symbolises the mundane life that is forever nothing but a haunting memory, reminding the vampire of their eternal torment. Thus vampires recoil from this reminder if all they have lost.
Bram Stoker included a dislike of garlic in his 1897 novel Dracula for no readily apparent reason, and the tradition has stuck around ever since.
It was originally garlic flowers that were used as a garland or a stuffing into the body cavities to prevent dead vampires from rising again. This was mangled by Bram Stoker to become the aversion to garlic bulbs. (This theory offered no explanation for why garlic flowers were considered a suitable ward against vampires rising.)
Garlic is a simple, naturally grown vegetable, and as such it repels vampires as a blight on nature.
In some Christian traditions, it is said that Saint Andrew (who Christianised Romania, traditional home of the vampire) gave garlic to humanity. Being a gift from a saint, it of course repels unholy creatures such as vampires.
Garlic was commonly found in households and so became a convenient method of warding a house against vampires, because it provided easy comfort for frightened peasants.
Ancient Egyptian tradition used garlic to ward off demons, and the vampire myth borrowed this idea from that older source.
According to Vedic mythology, garlic was produced by a combination of a drop of Svarbhanu's blood and the nectar of immortality - where they fell to Earth the first garlic sprouted. Thus being a combination of demon's blood and nectar of immortality, garlic protects against psychic attacks and wards off demons (and vampires).
Garlic was rarely used in Russia because it didn't grow well there, so people there never developed a taste for it, and Russian legends about vampires adopted their revulsion to the herb.
The indigenous people of tropical America used to use "bat-bane" wreaths of aromatic herbs hung around their necks to repel the real threat of vampire bats, and these wreaths actually included garlic (allegedly, presumably after Europeans brought it over from the Old World). Any causal link to the European vampire traditions is tenuous at best, but I included this here because the source is an article in The Medical Journal of Australia, albeit a "Christmas Competition" article which reads as satirical and is quite amusing.