h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

After reading a recent article
http://news.bbc.co.uk/1/hi/health/6268585.stm
regarding human parasites and MS progression, I was wondering what the situation is with human parasites in countries like the UK, not specifically in relation to MS, but more generally.

Are there [m]any common parasites which are sufficiently non-symptomatic that someone could be chronically infected without ever knowing it?

I suppose some gut parasites might fit the bill.
I guess toxoplasma gondii could qualify as a more 'internal' one, but would there be many others?

They certainly exist and I believe there are many with a capital M.

I certainly know of a tick that lives in our eyelashes but does no harm.

With the myriad microbes around on the planet (their biomass vastly exceeds multicellular life), it is basically impossible for a creature to exist without being continously invaded by such things. That is why we have such a flexible immune system geared to recognise just about anything foreign and stamp it out as quickly a possible. These things evolve all the time and we must evolve with them.
I would say for a parasite the perfect evolutionary equilibium is achieved when the host is not harmed noticably, thereby maximising their reproductive chances and hences survival.