Consider the following possibilities:
Someone sues over election results because a parasite changed the outcome according to a computer
model of the human population with and without the parasite.  The legal claim is that the parasite doesn't
have the right to vote or to modify the outcome of an election.

If you find that you have a number of such parasites and you expunge them all from your system.  What if
your personality changes significantly and you have to cultivate a new set of friends?

Consider moreover that it could be found that certain parasites are more common in cities and others are
more common in the country areas.  Picture that antidotes are placed in the water supply and country folk
personalities change to becoming more liberal and city liberals become more conservative.
Blue states start voting red and red states vote blue.

Picture that a fanatic belief in Islam is tied to a parasite found in camels and related rodents.  Picture that
a cure to Al Qeda is found and placed into the water supplies in Mecca.  Those visiting during the Haj
become less devoted after drinking the water.
Medbot
asking and answering medical questions