If in the omniverse there are only 10 pathogens, and Bruce living in this omniverse is healthy,
either he's naturally or artificially vaccinated, or
Bruce hasn't been exposed to all 10 pathogens.
The reality appears to be Earth is awash with pathogens, many of them capable of sickening a human.
This source says there are far more than "a dozen". https://www.dph.ncdhhs.gov/programs/epidemiology/communicable-disease/a-to-z