It is up to the host to help you in this problem, if it is a virus of some sort then that is your hosts problem... they should be the ones fixing that sort of problem.
If the problem is a script on your site, then their server logs will tell them the exact script(s) and what paths on the server to look at.
IMHO you should put the ball back in their court and push for help as you are paying them for a service which includes technical back up, a host can not sell a service unless they also provide technical assistance and help when requested, so if they have a ticket system or a contact centre, start a service ticket, email them or call them.
I have had various run-ins with hosts in the past who are all too ready to take your money but as soon a SHTF, they bail out. I could tell you about a host who failed to secure their server, tried to blame me and tried to charge me £140 and demanded payment before I could use the customer service ticket system for assistance.
Another host has such poor backend security that a website of mine got destroyed and again, the host tried to blame me until I pointed out a few facts and made it clear that I had just recently finished a Network Administrators course and I wasn't your average confused mullet brained client and that they need to pay attention to the facts that they still tried to dispute with Trading Standards, this was another hosting company who wanted to charge me £79 to rectify the problem, strange considering that was the cost of a years hosting, so what they wanted me to do was start a new hosting plan... not fix the problem.
Most important thing to remember is that the web hosting market is full of cut throat vendors who are eager for business, so your host as they currently stand are in a precarious place, you have the option to port your domain to another host, they may want a release fee for letting the transfer through but the receiving host would do most of the work because they will if they want your business.
Companies need reminding that times are tough at the moment and that puts the customer back on that pedestal of "Customer is King" again, companies need reminding of how they generate income to pay people in those jobs.