Retribution/Plague Marines/Strategy
Overview
Plague Marines are servants of Nurgle. The original Plague Marines belonged to the Death Guard Legion before the Horus Heresy, but any Chaos Space Marine that dedicates himself to Nurgle can become a blight-infested hulk of disease and ruin that is a Plague Marine. Because of the corruption rotting away the nervous systems in their bodies, they feel no physical pain.
They possess a missile launcher (which does equal damage to the Tactical Marine missile but also has a unique snare effect on vehicles that slows their movement speed) so that you can use them as an anti-vehicle unit, and bolters that infect enemy infantry with disease, damaging them over time.
First of all, they are a very solid fire base. Their bolters are not very powerful at first glance, but they do damage over time so are effective against every kind of infantry. The DoT is effective at killing fleeing units as they will continue to take damage as they fall back, even when out of range of the Plague Marine bolters.
One of the most significant aspects of this unit is that they are very, very durable. Each time a model dies it explodes, healing the rest of the squad (and any friendly units around). They can be suppressed but it only slows them down a little bit and doesn't affect their shooting - they can just walk inexorably towards the enemy like zombies. They have excellent health regeneration as well. As a result of their health regen, you won't need to fall back with them as often as you would with other units as they can heal on the go.
They function best as a shield unit due to their high durability, but they can also be rather handy in melee too - if your opponent has superior firepower, put them into melee stance and go in for the kill. When they die, they explode, damaging nearby enemy units in melee range and healing your troops, massively increasing the staying power of your forces and making a mess of enemy melee units like Howling Banshees. Their aura of disease also slows down enemy units in melee range as well, so they can be a good way to crowd-control melee units as well.
Because they do not require set-up time, and also because of the snare effect of their missiles, they are extremely effective at pursuing fleeing vehicles (or large monsters like the Great Unclean One, as well as Terminators, as their missiles are highly effective against super heavy infantry like Terminators). Keep them moving as their missile launcher reloads, and only stop briefly to fire, then keep moving.
Their counters are various forms of artillery - Plasma Cannons, Manticores and Noise Marines usually wreak havoc on them - as well as plasma gun-equipped Tactical Marines and Tzeentch-dedicated Chaos Space Marines.