Chest folder is a password protected folder. avast lock the infected files there and virus code cannot do nothing unless you ‘recover’ the file to your system manually. When a virus is found, if you do not know how to react, the best is send the file to Chest. From there you can resend the file to Alwil for analysis (virus@avast.com), delete the file, try to repair, etc. You’re safe using the Chest, that is why the Chest exists.