As Infected-Try to separate them and focus on one guy until he goes down. Spit on the down players. Try to death charge players causing insta death. Combo them. Try to get good boomer vomit hits.
As Survivors-Run. Don't stop moving, the longer you wait they faster they spawn and come. Auto Shotgun. Pills. Tank is the number one priority, light him on fire. Share pills. If someone is down use Adrenaline, they will be up very quickly. Constantly check behind you, throw Molotovs behind you in choke points. If boomed on, throw a pipe bomb. Boomer bile combo'd with a Molly is a great way to take care of hordes.