There is no such thing as spells. The reason that "spells" seem to work is that the person being cursed is aware of the curse; and, is a true believer in this kind of witchcraft. The person being cursed may even sicken or die.
However, the reason this spell "works" is because the cursed person causes these illnesses to develop within their own bodies through their own mind power. The mind is a very powerful organ and cause all kinds of illnesses, sickness, disabilities, etc. through only the power of its own suggestion.
These people who appear to fall victim to these spells believe, truly believe, that this curse will work; and, thanks to their own minds working against them, the spell seems to work.
In reality, neither the curse nor the one placing the curse have had nothing to do with the other person's disaster.