How much secure is cloud computing

I heard that Cloud computing is a term where a company use the third party resources instead of running software's applications on local server or storing data locally. clouds are hundreds or thousands of computer connected via each other and shared by companies to host their application and manage data.
But I have some confusion about the security in a cloud environment, please expert clear me about my doubt, my questions are.
1. Is the data center where that computer is located staffed 24 hours a day, seven days a week? What staff is on-site?
2. Are servers dedicated to each customer, or do multiple customers share a single server?
3. Is there any disaster management, if something goes wrong with server then how we get our data there will be backup data on different server?
4. How many chances there that our data on a cloud server get infiltrated or infected with virus.
