Another discussion crossed my inbox that I thought I would share.
Files are scanned for viruses after they are uploaded. If a file is found to be infected, a property is set so that users can't download the file from the browser or sync the file in the OneDrive for Business client. Note that files larger than 25 MB in size are not scanned, as well it can take up to several hours for the scan to hit any particular file.
For that reason it is recommended that your enterprise have strong endpoint protection in your security strategy. For further information read this article
Important: These antivirus capabilities in SharePoint Online are a way to contain viruses. They aren't intended as a single point of defense against malware for your environment. We encourage all customers to assess and implement antimalware protection at various layers and apply best practices for securing your enterprise infrastructure. For more information about strategies and best practices, see Enterprise Security Best Practices.