Usog is a Filipino superstition that attributes an illness to the greeting of a stranger. It is believed that young children are susceptible to usog.
If after encountering a stranger, a child develops a fever, the stranger is sought out and asked to wipe his or her saliva on the child’s forehead, chest or abdomen.
Filipino parents worry when they catch a stranger expressing fondness for their child or even just looking fondly at their child. If the stranger senses this, the stranger will sometimes say Pwera usog… (“excluding usog“) meaning he/she understands that the parents are worried of usog.
If parents get really anxious, they will ask the stranger to lawayan ang bata (place saliva on the child) para hindi mausog (in order not to be victim of usog).