In my fictional world, we have our John Doe (main character), an ordinary man who happens to be given a drug in the initial stage of testing that gives the user super-human immunity to biological viruses, bacteria and fungi. As an example of John's dramatically improved immune system, he is not affected by any current or in-development biological weapons (such as weaponized Plague or Anthrax), and common illnesses (Flu & the common cold) show no signs in his body.
To add to his advantage, no super-secret military or government organizations are aware of John's otherwise high immunity, so they are not pursuing him for his DNA. So what would be a good profession for John Doe, in which he can take advantage of his super-immunity?