The Plague

The plague was an illness that spread through the Clans — starting in WaveClan, moving to MeadowClan, and eventually spreading to MarshClan — based on FIP.  It was eventually ended when Poppyblaze, Cinderspots, Woolflower, Shadesong, Claywillow, Loamstone, and Moonpaw returned from their journey to the Tribe of Stone Mothers with the herbs to treat it in tow.
In WaveClan, cats lost Ottersplash, Blazeheart, Scalefang, Branchpelt, Hailstrike, Cliffrunner, Cloudpelt, River, Ivyshade, Coalmask, Ridgeclaw, Minnowleap, Flowerflight, Stagleap, Sandytail, Ashstorm, Stonebreeze, Lightningheart, Primrosepelt, Doeskip, and Pantherheart.
In MeadowClan, cats were left mourning Deerfoot, Rushtail, Lilywater, Milkfur, Catchtail, Fogfur, Ashfeather, Orien, Tansyfoot, Gooselight, Blackclaw, Viperfang, Turtlespots, Lionflame, Yewberry, Leafbreeze, Weaselpaw, Gorsestorm, Granitefall, and Flowerdawn.
In MarshClan, cats grieved Talonfeather, Streamfang, Otterleap, Breezeflight, Russetfang, Cricketsong, Stormstrike, Sageflame, Rapidclaw, Petalkit, Daisyflower, Darkfang, Tallshadow, Lizardtree, and Pebblefall.
The leaders of the Clans had their own casualties: Halfstar, Reedstar, and Jaggedstar all lost two lives to the illness before their ancestors were able to cure them.