These could probably be the left anone infection that couldn't be deleted from an infected CD/DVD that was accessed on these computers. In such cases, the "Infected" flag is stored in the clients's database file on the client computer (not SEPM database). This type of "Infected" flag can be cleared only by inserting a clean media in to the same drive (on which the risk was reported as left alone) and performing a full scan on the clean media.
Note: 507 still infected count not necessarily mean 507 machines. It is possible that many of them are from same computer. So, I would suggest finding out the total number of computers first. you can easily get the still infected list by clicking on the "Still Infected" count under "Virus and Risks Activity Summary" on the Home page of SEPM.
If the number of computers is too many to work manually, please let me know, so that I can give you an alternate workaround.