In the Aztec tradition it is believed that the "evil eye" (mal de ojo) occurs when someone looks at babies or children and, on purpose or not, affects them in a negative way, i.e. some babies might get headaches or some fever for no apparent reason or they just might be scared of that person, so the ojo de venado protects them from those looks and their negative effects on them. Of course babies will always get the attention of everybody and parents cannot keep them from being looked at, so, from our perspective, they have to have the ojo de venado to protect them.
This can happen to babies more often because they are very fragile and do not know how to defend themselves, but this can also happen to teenagers or adults, thus the necklaces or bracelets with ojos de venado that some of us wear.
I know that some people, especially of European descent think that this is all bullshit, and everybody is entitled to think whatever they want. In my case, after having seen a lot of stuff and after realizing how important and especially how wise my Aztec, Maya, Zapotec, etc ancestors were I totally believe it (over the years, rituals that the spaniards told us were bad and forbade us from performing, have proven not only true but also healthy, and in turn it is the spaniards that have proven really stupid with all the crimes they have committed against not only us and our culture but also against our mother land).
Some other, non-European cultures have the same objects that we do, for instance, Papa Zak (my West-African dance teacher) wears a bracelet with an ojo de venado too, he told me that this bracelet helps him perform muslims/african rituals (he's a muslim priest or something like that, don't know the exact word or title to use though).
