It could be hygiene.
Some kids are very handsy, they don't wash their hands, they touch things/the ground/the walls/other kids or anything, and then their mouth/face etc. And viruses live on surfaces for awhile.
Our Pediatrician said, that for kids up to about 7-8 years old, they may get sick a lot. Especially in school. Some more than others, because they are so, touchy and handsy etc. But after that age, they are less impulsive and more hygienic. And are more careful etc. Our Pediatrician, is also a specialist in Epidemiology.
Last year, my son got sick a lot. Practically every month it was something. But a different virus each time.
But just go see your Pediatrician.
Ask him/her.