My experience parallels so many here.
When I smoked, I'd get three to five colds per year, and about every other year, one of those would turn into bronchitis.
When I started vaping, I came down with a case of "quitter's flu." After my respiratory system cleared out from 47 years of three packs per day -- took several weeks -- I haven't had a single cold. It's been almost four years. And I'm ecumenical about vaping: I let smokers -- even those who are obviously coming down with something -- take hits off my rigs. And I never clean drip tips.
If you're using any significant percentage of propylene glycol in your e-liquid, remember that before the invention of micropore filters, many hospital ventilating systems (especially in ERs, ORs, ICUs, neo-natal wards, etc.) pumped lots of propylene glycol into the rooms. The stuff is an antibacterial and antiviral.