Colic

I saw a young Australian couple traveling with an infant. A placid sleeper in Australia, the infant had been screaming off and on since arriving six days earlier, attracting complaints from other guests, driving the poor parents to desperation. They wanted to go home.

It was colic, a surprisingly common affliction of healthy infants. In theory, they are suffering abdominal pain, but all tests are negative, and none of the innumerable treatments work well. It disappears after a few months. I wrote them a note.