How safe is clients’ data in cloud computing with public internet platforms?

The verdict is out and there is a consistent push to make cloud computing the number one choice for small and large businesses due to the convenience it provides. What I want to know is, what are the chances of someone intruding, intercepting, or infecting the system with a virus when cloud computing is hosted via public internet platform? Does anyone know what kind of antivirus system cloud computing companies use to protect the data of users?
