Other forms: crusaders
A crusader is a person who works hard or campaigns forcefully for a cause. Most crusaders advocate dramatic social or political change.
You can call a fierce champion for a cancer cure a crusader, and another kind of crusader could be an activist who works for school reform. Crusaders tend to be radical or at least progressive, embracing some kind of change. Crusader comes from crusade, which meant "campaign against a public evil" in the 18th century, but which earlier referred mainly to the religious-based military Crusades of the Middle Ages.