I guess Mike infected me.

You're talking to someone about a disease you had a few years ago.

A: So you were really sick, huh?
B: Yes, I was. I guess Mike infected me. (Needless to say, B was infected before he "was really sick"

Why not use the past perfect here? I feel as if this were a story and not a conversation, the past perfect would be better.

I was really sick and I couldn't move. Mike had infected me.

Here, the past perfect sounds much better to me than in the first example, but I still might use the past simple if I'm not paying attention.