Delete the malicious files on the site/hosting

Viruses on the hosting!

The virus appeared on one site out of many, I removed it and two days later he appeared again, on what the hosting provider has responded limitation. The contractor who had previously removed it said its possible the virus sits in the files of other sites and need to separate them. All sites are on the same account.

Task: to check all the hosting for viruses, to find the source of infection, to remove all malicious files and it is important that after a time he appeared there again.