We had this problem when multiple websites were hosted at the same account, such as:
/home/site1/…
/home/site2/…
If one of the sites has a “dirty” compromised PHP script – it could get to all clean sites and infect them.
To solve it either open separate hosting account for each domain or buy “reseller” account from hosting provider. There are possibly other ways to physically separate hosting spaces between domains. Reseller account likely to be more cost efficient.
Mike
