It cycles seasonally, but it's for social reasons. It cycles in the winter because of holiday traveling spreading local outbreaks, and back-to-school season when all the kids go back to be concentrated in 1 place.

Back to school is also why there's a late-summer bump.

Winter also has people more concentrated indoors in colder climates, and summer does the same in hot areas. Texas had a HUGE outbreak a month ago because everyone was huddled indoors to get out of the heat while the kids were bringing it home from school.