The Contagion is a Daemon Engine of Nurgle and is an over glorified trebuchet and a ripped off of the ratman's own shit throwing artillery piece called Plagueclaw Catapult. Like the Squig Catapult, it seems that in the grimdarkness of the 41st millennium, catapults still find some usage from the forces of Chaos.
The Contagion's main weaponry is the trebuchet which flings diseased shit/dead people/used up adult diapers/your sister's tampons/vomit/more shit at their enemies. While highly inaccurate being a trebuchet from the medieval ages and all, the main job of the Contagion is less on a direct hit and more on the spread of Papa Nurgle's Christmas gifts. While the trebuchet is used for long ranges, at close range the Contagion is far from defenseless as it mounts a forward pus cannon which sprays a stream of highly infectious diseases and parasites.
To make matters worse, the Contagion is made up of wood...well....magic wood at least that has been blessed by Nurgle. What you literally think that Nurgle is stupid enough to send in normal wooden engines of war in an age of lasers and rockets? Anyway this blessed wood is as tough as ceramite and is known to deflect multiple Bolter shells and even Battlecannon shells.
The only downside of the Contagion is its slow as molasses speed and having no defenses if assaulted at the side or rear. The Plagueburst Crawler might just be its spiritual successor.
Tabletop EPIC Rules
Stat-wise, the Contagion are a formation of four for 325pts (the same price as a Basilisk formation) and are an armoured vehicle with 15cm move, 5+ Armour, 5+ CC, and 5+ FF. This makes them slightly slower than the comparable Basilisks, but with a better CC score. Both pretty much irrelevant stats when it comes to artillery pieces.
Weapon-wise, the Contagion has an indirect firing 45cm 1BP weapon with Disrupt, and a 30cm AP4+/AT6+ weapon with Disrupt. This means that the formation can throw out two templates with Disrupt to a range of 90cm, along with some token short range firepower, making the additional blast markers give by Disrupt a trade off for the Basilisks increased range.