Why do we never share germs?

Nurses and doctors of Glow! A question for you!

So, my husband and I have had a pretty decent track record of getting some yucky Illnesses in our 3 years of marriage: influenza A for me, and currently on vomiting bug #3 for him (which seems high considering we don’t have/work anywhere near kids or healthcare). Plus a myriad of colds, strep throat, etc.

But (knock on wood) we NEVER catch it from each other! During my influenza bout- I was sick in bed sweating and coughing blood for a WEEK- and he was there the entire time. He insisted on kissing me. He never caught it. We both had our flu shot that year so maybe that’s it but still. During his stomach viruses I refuse to kiss him once symptoms onset (hellooooooo emetephobia👌🏻) or really touch him, but he’s a big baby who needs me to sleep in bed with him no matter what so I concede despite my protests to take the couch (honestly anything to keep from hearing him throw up or risk it myself 😭). And we are super kissy before any symptoms so it’s not like in the 5 minutes before he starts throwing up I’m missing those germs.

But why do we never catch it from each other? Again - LOTS of knocking on wood as he’s currently laying next to me still recovering from norovirus that started last night. Is it just luck or is this a science thing?