If you have a rooster and he mates all the hens, the bacteria might get passed around. But I don't think it's a real danger. The only danger is to the hen that has the infection, and it isn't something that will just clear up on its own. Left untreated, it will eventually cause sterility, and sooner or later, death.
I do not advise treating all the hens if you can't identify the victim. Best thing to do is try to monitor who is laying. Salpingitis usually causes a hen to stop laying eggs. And if you can associate a hen with having just passed more of the lash material, that's going to be the best way to know which hen is infected.