On-demand and on-access anti-virus scan requests are distributed over a plurality of virus checkers for on-demand anti-virus scanning concurrent with on-access anti-virus scanning. For example, the on-demand requests are grouped into chunks of multiple requests, and the on-demand and on-access requests are combined in a queue. Scanning for requests from a prior chunk are completed before distributing the requests for a next chunk. To give priority to the on-access requests, the on-demand requests are not placed on the queue unless the number of requests on the queue is less than a threshold. An on-access request is placed on the queue in response to a request for user access to the file. On-demand requests are produced in response to a request from a system administrator for anti-virus scanning of the files in a specified file system.