Open the Variable editor, create a new unit group variable. Create a new trigger so that when a unit is attacked/has a spell cast on it/whatever the event is that would normally trigger the disease buff, have it add them to the unit group.
Then create a new trigger that checks periodically, every .1 seconds or whatever, if every unit in the unit group has the Disease buff; if not, have it remove them from the unit group.
Then for your main trigger, make the condition = the unit is in the unit group, rather than has the disease buff.