Definitely. The first is a weakened response; the second is a more persistent attack.

Edit- sorry, I missed the possession part. I have witnessed a spiritual possession before, and I had no reason to believe any parasites were involved. But I'm sure that a possessed person will tend to engage in vices and often become alcoholics or drug addicts. And people that are physically ill can become mentally ill as well. It's fairly easy to observe in the homeless ... mental illness is prevalent and it often seems that they are talking to / afflicted by spirits. Which came first is anyone's guess. My opinion is that it can start from either end, but either problem will make a person more prone to become a victim of the other.

Definitely. The first is a weakened response; the second is a more persistent attack.