A Theory on Disease: Disease Load and Pathogen Diversity

Apart from the environmental toxicity, I feel that there is another component to modern sickness with parasites.

While people in the past often lived in small geographic areas the diseases they contracted were all local and of common strains? While they were probably exposed to high levels of pathogens they could handle the burden because they had common strains.

My question is, is it the diversity of modern pathogens like worms, Borrelia, etc that cause major symptoms within a person? You could touch a doorknob in a northern city like Oslo, a climate which normally would expose a person to low levels of parasites, and contract parasites from Asia or Africa.

I know that most pathogens never leave the body, and each strain often has a specific niche (i.e. Borrelia strains), so is it possible that the more diverse a person's pathogen's are (through extensive travel, or interaction with people in cities), the more likely they are to be sick?