Text interpretation, need your help!

Ladies, I've been dating a guy for a year and things have got better and better each time. It's a distance relationship so it's a bit different than what it would normally be but its ok :) he doesn't normally express feelings but yesterday he said: 
Him: is it bad if i think about you with all my spare thoughts?
Me: well, if it's bad I think we both caught the same disease...
Him: I hope it's more of a parasite!
What does the parasite reference mean to you? Thanks!