What does "Disease" Keyword mean?

For example, how does a spell, which gives Disease immunity, work exactly? Does it just block all the Corrode damage or spell(s) having Decay keyword? By the way, I'm playing Druid now, and I noticed that most of useful damage spells of Druid target enemies' Fortitude, while few Druid spells can decrease the Defense; Actually, 4~5 Druid spells only can cause either Might or Constitution Affliction, and those spells even check Accuracy vs. Fortitude!

Is there any good way to use such Druid spells effectively? I have no idea what equipment or multi-class would fit Caster-oriented Druid except a staff which increases Plant and Beast Power Level. Thanks in advance!