How do you keep yourself from getting sick?

I wear masks, I wash my hands, I'm not going out partying everyday. I keep getting sick. And not a light, gone in a day sick, but actually non-stop coughing tummy rumbles cant leave my house kind of sick. I feel like I spend more time sick than healthy these days.

Am I missing something? Does everyone else experience this too or is there some crazy liquid everyone is drinking to stave off the germs? Or staple super vitamin food im missing out on?

I thought I had a great immune system until I moved to Japan. Do I just have to keep getting sick until I build up an immunity to Japanese Germs?

(any actual advice is VERY appreciated!).