These organisms have been around for millions of years.

Two things to keep in mind, people around the world are infected with these organisms, and in many cases have developed a tolerance/balance with the parasite as well as bacteria and viruses. Keep in mind a successful parasite, does not kill its host as it depends upon it continued existence for it’s own survival. In some case these organisms are almost commensal in nature (non-pathogenic or non-disease causing) as long as the person/animal is in reasonably good health. It is often when the human/animal becomes infected by other organisms, becomes malnourished or it’s immune system is not adequate (age: too young/too old, other causes), to maintain the balance, that these organisms multiply and in of themselves become health hazards.

The other issue is the increase in population and changes in land use (from natural to farms, towns, etc.) all of which will change the water table (redistribution) and quality of water. Also, (not a political commentary) as populations become mobile and immigrate or emigrate from various parts of the world, susceptible populations become exposed and subsequently pass on many of these infections.

Pete