The two species have a lot in common. Most humans believe that they are humans.
Many cats believe that they are humans, too.
The rest of the cats refuse to lower themselves to the level of mere humans, believing that they are superior beings. The calendar said that cats will instinctively try to hide their pain and discomfort. In the wild, animals that are sick are easy prey, so cats instinctively hide any illness. It's a time-honored tradition among humans everywhere. It's a highly-refined form of ritual lying.