accidents

Respected scholars, As-salamu `alaykum. I work in an all-Christian environment, and whenever someone has an accident or is ill they tell me this is from the devil or the devil has caused it. Are we Muslims to believe that too? I know that in the Qur'an the Prophet Ayyub (peace and blessings be upon him) says that his illness is from the devil (mass min ash-shaytan). I'm very confused about this issue because in other verses it is mentioned that when bad things happen it is because of the bad deeds of humans, and in others that it is a test from Allah. So could you please clarify this? Thank you.