Happiness is a stat bar exclusive to the Santa's Little Helper scenario.
The happiness bar will tell the player how happy people infected with the Neurax Worm are, and replaces severity as the middle bar. It has a yellow color. Certain things will bring up happiness, including the symptom combos On The Naughty List and Ultimate Christmas, and most of the symptoms, save ones from the Sloth and Indulgence trees, the only traits which decrease happiness in the scenario. Transmissions do not increase happiness, however, but some abilities, like Red-Nosed Guide and Red-Suited Pilot do.
The Happiness stat bar replaces the normal Lethality bar.