I run a whole army of Death Gaurd led by Typhus with 3 10 man squads in rhinos and I can tell you they are not invincable.
Though the armys record stands at 17 wins 0 loss and 1 draw the ames are not a walk in the park.
The way to beat them has been stated in posts above hit them with melta and plasma and they go bye bye.
But one thing people tend to miss is their Initiative is only 3 charging in with some power weapons ( and yes bring a fist of some kind) or with Monstorous Creatures is bad news for the Plague boys.
For you gun line armies like Tau and Gaurd it gets even easier.
Max range for Plagues is only 24" stay back and hit them with lots of shots every turn and don't spread out the shots either concetrate fire on one squad at a time get rid of it then repeat.
If it takes you one turn of shooting to wipe out a 10 man squad you are still removeing close to 300 points worth with upgrades.
The West Coast Knight