I'd say the first couple of years, I got many more minor illnesses than I'd have experienced previously. Nothing serious, but on a par with when my son was at school and brought home every cough and cold going. Then - I think - my immune system beefed up, and for the last 3 years I've barely had a cold.
I eat very very healthily and take a prescribed multivitamin daily, so that undoubtedly helps. If you're getting frequent illnesses that you didn't get before sex work, defo start taking a multivit, I'd suggest getting Foreceval. It's not on the shelves but it's available at the pharmacy counter if you ask. If you menstruate regularly, also take ferrous fumarate (again available without prescription OTC.) You won't see immediate results but if you track your health daily, you'll see the results in 3 months. And summer is definitely the time to start since so many bugs are virulent in winter.