A well-known defense against vampires is to chew garlic and have cloves of garlic around you at all times.
Of course chewing garlic release a putrid smell that is almost impossible to get rid of.
This is because upon chewing garlic a sulphur containing chemical called allicin is produced, and sulphur containing compounds (thiols) are well known to smell nasty.
The smell is often associated with rotting eggs or rotting meat, and it this is exactly what people wanted as surrounding oneself with this smell is thought to offer protection against the evil vampires.
The Romanians took this so far as to smear garlic on every nook and cranny (including their windows, doors, gates and even cows).
Now the question begets, "Why is Garlic useful against vampires"; wouldn't it be more logical propagate a myth where vampires are terrified something that smells good? If not, something that smells less vile.
One of the possible explanations for this odd belief is that garlic is known to have anti-bacterial qualities.
Garlic was used to ward off various diseases such as the plague and it is believed to have many health benefits.
In fact, there have been many scientific studies (1)(2) showing the anti-bacterial properties of garlic.
As vampires were associated with "evil", a category that diseases and plagues belong to, one might assume that garlic might also ward off this evil.
Another hypothesis is that garlic is natural mosquito repellent.
Since vampires are also blood sucking fiends, having an odour of garlic might repel them away also.
Vintage arsenic poison bottle A new and rather hypothesis (3)(4) that recently is that the burning of arsenic compounds started off this whole trend.
This might sound a bit preposterous, but there is certain logic to the argument.
During the time vampires lore is dominant; Arsenic, a potent poison, was believed to hold power over evil.
This stems from the ancient ideology that one must fight evil with evil, and what better evil to use than a potent poison.
This ideology as it turns out isn't too absurd as arsenic based compounds do have certain anti-bacterial properties.
This ideology was strongly held by many alchemists in major cities like Prague and Moravia at that time.
When alchemists were hired to ward of vampires, they would burn arsenic containing compounds to provide the illusion that they are warding off evil with powerful evil slaying odours.
These arsenic containing compounds when burnt would provide a nasty odour that smells exactly like that of garlic.
The peasants would of course notice this and make the connection that chewing garlic would produce the same effect and would be cheaper than hiring an alchemist.
This homebrew is extremely cheap and easy to reproduce and thus most likely spread very quickly among the peasant population.