(Ok, this post sorta got out of hand and became huge...)


A vaccine is actually a sterile or already dead biological lifeforms (a pathogen) put inside your body directly into it's defense HQ (lymph node) alongside a fairly large dose of white cell stimulus. In which case the immunitory system responds in force, eat the percieved threat, anaylses it and basically create a police profile of it and antibodies to neuter it when found.

But I do understand what you are referring though. For exemple, nurses generally have excessively powerful immune systems because they are perpetually facing "germs" that are in their weakest states (a recovering patient) over exposed periods of time and thus creates very exhaustive "profiling" of the pathogens.

ON THE OTHER HAND

A toxin or toxicant is a protein that causes a chemical reaction, which is another thing altogether (poison/venom/drug/etc.) If a chemical is nocive to your body, it will damage it.

Generally speaking, if a particle can hurt hurt you it means they are particles that your body can't deal with/filter in the first place and lingers for long periods of time (ex: crystal meth) or basically forever (exemple: heavy metals such as lead or mercury) and having regular small doses over time just means you progressively accumulate them until they become a large enough dose to do bigger damage (progressive overdose and lead poisoning), and eventually, death.

Considering the backstory, I doupt it is constant abuse of illness and disease but probably more about drugs and poisons. Under controlled circumstances some toxins "abuse" can create drug tolerance (the body reacts less to the effect, but it doesn't nessessarly means the negative effects aren't there, but not percieved) and/or addiction. (Exemples: bee stings reduced effectiveness, morphine habit or pygmys being highly resistant the poison they coat their blowdarts with). But the opposite is also true, where the effects can actually become amplified, or devellop serious allergies.

In short, a toxin is toxic exactly because they are the things that your body cannot normally filter. Abuse over the course of many many many many generations of toxins your body eventually dispose of can mutate some sort of makeshift filtering (like alcohol in european lineage compared to amerindians), but more generally, it only screws up with the lineage (exemples: Hereditary problems or Atmospheric Pollution, which contains plenty of tiny tiny tiny doses of harmful materials that your body never disposes off and lingers in your blood, is transfered to your children, which is born with a small dose of pollution from you, which itself will live in a polluted environment, which adds the same toxins, which accumulates, give to it's own child, etc. creating more and more problems at every step [see: mexico])


What constant usage of toxicants COULD/WILL do to benefit you, however, is knoweldge of what toxin does what effect, being able to quickly diagnose what is ailing you (potentially the tolerance the give you a little more time to react to it), and being able to choose the correct antidote to use against it.