"Nurgle's blessings will be released into the atmosphere ensuring Complete. Global. Saturation."
- – Plague Champion Wesker to Ultramarine Captain Redfield during the war.
The Plague Wars were a series of conflicts between the forces of the Plague God Nurgle, led by the Traitor Primarch Mortarion, and the Imperium of Man, particularly the Ultramarines Chapter and descendants, led by the Lord Commander of the Imperium, Roboute Guilliman. The conflict arose from Nurgle's ambition to expand his domains to the Ultramar planets, taking advantage of the then ongoing Indomitus Crusade and the fact that the Cicatrix Maledictum had obscured the realm from regular warp travel. The start of this campaign marked the end of the Indomitus Crusade, and it is the one of the major background events for the Dark Imperium novel and the Fate of Konor campaign.
Nurgle starts seeding his new garden
And isn't THAT just the most horrifically disgusting title you've ever read. In~uen~do!
Way back before the 13th Black Crusade had started, the Chaos God Nurgle was trying to absorb the Ultramar Realm to his Garden. He promoted and favoured small cults of pestilence and corruption to grow and prosper, unbeknownst to imperial authorities. Starting from neighboring Imperial systems to the galactic North of Ultramar, now known as the Scourge Stars, small nurglite incursions tried to spread the plagues to the rest of the systems, to little effect. It wouldn't be until the start of the 13th Black Crusade and Roboute Guilliman's Indomitus Crusade that the scenario would be right for a Nurglite campaign to become a real threat.
The main forces leading the Nurglite forces were the Death Guard Legion, led by the Traitor Primarch Mortarion and Typhus plus a major force of daemons under the Great Unclean One Ku'Gath. These forces could not launch a direct invasion on the core planets of Ultramar due to its massive defenses and fortifications, not to mention the infights between the three chaos leaders, who each wanted to gain Nurgle's favours for themselves. Thus, they started a campaign of fear and attrition on nearby worlds, attacking and corrupting nearby agri-worlds, forge worlds and hive worlds. Taking advantage of the situation, Nurglite forces launched plague after plague unto these planets, ranging from zombie plagues to nurglings exploding from inside the victim's body, helping to grow their forces.
When news of escalated aggression made it to Roboute Guilliman,
he dissolved declared the first phase of the Indomitus Crusade over and launched his forces to reclaim and rescue the Ultramar Realm. This may sound foolish but you have to remember that Ultramar is extremely important to the Imperium. A mere five hundred worlds may be insignificant numerically, but even ignoring the spiritual importance the realm's been largely untouched by the degradation of the Imperium and so is a vital part of keeping the Imperium's knowledge from fading away, which would doom humanity in a few short millennia.
The Imperial Counterassault and the other Chaos Gods ruin Nurgle's fun
After Guilliman arrived and surveyed the direction of the war, he decided to first try and cut Chaos reinforcements from their origin, since the forces of Nurgle were still too great in number for the arriving imperial fleet. Cutting chaos forces from the Scourge Stars meant that reinforcements would cease to arrive, a hard blow for the Nurglite armies. Aiming to destroy the source of the undead armies, Guilliman's forces attacked and destroyed most of the Greater Daemons and Daemon Engines, stopping the enemies's cannon fodder from appearing.
After a massive series of imperial counterattacks, the remaining Chaos Gods took that opportunity to invade the Scourge Stars, which meant that the Death Guard and associates were forced to retreat to the Scourge Stars to defend and fortify the Plaguefather's foothold in realspace.
The aftermath: spring cleaning Ultramar
After the war was over, dozens of planets in the Ultramar Realm were horribly contaminated by plagues. The imperial authorities oversaw the purging and rebuilding of these systems; once that was over, Ultramarine forces launched a series of attacks to nearby Chaos-controlled systems. The Death Guard, still in realspace and once finished with the defense of their worlds, decided to take a stroll through Tau-controlled territory, which became the most threatening campaign for the Tau since the Damocles Crusade.