Usog or balis is a topic in psycho-medicine in Filipino Psychology (but considered just as a Filipino superstition in Western Psychology) where an affliction or psychological disorder is attributed to a greeting by a stranger, or an evil eye hex. It usually affects an unsuspecting child, usually an infant or toddler, who has been greeted by a visitor or a stranger.
In some limited areas, it is said that the condition is also caused by the stranger having an evil eye or masamang mata in Tagalog, lurking around. This may have been influenced by the advent of the Spaniards who long believed in the mal de ojo superstition.
Once affected, the child begins to develop fever, and sometimes convulsions. Supposedly, the child can be cured by placing its clothing in hot water and boiling it. In most other places, to counter the effects of the "usog" the stranger or newcomer is asked to put some of his or her saliva on the baby's abdomen, shoulder or forehead before leaving the house. The newcomer then leaves while saying: "Pwera usog... pwera usog..." The saliva is placed on the finger first, before the finger is rubbed on the baby's abdomen or forehead. The stranger is never to lick the child. The practice is that the stranger or visitor is asked to touch his or her finger with saliva to the child's body, arm or foot ("lawayan") to prevent the child from getting overpowered ("upang hindi mausog").