With harassment, dirty breaths, water down your throat through your nose any advice on how to avoid getting an infection in your lungs?

The only thing you can do to avoid infection is to take care of your body by sleeping, eating properly, and maybe supplement vitamin c to boost your immunity. Technique wise I always recommend placing the tip of your tongue on the roof of your mouth to act as a kind of splashguard when you are breathing through the snorkel on buddy breathing. This will reduce the amount of water you might aspirate while breathing through the snorkel.