I’ve run Kasperky online scanner and a file was infected with Trojan-Downloader.Win32.Small.bke.
I open Windows explorer to scan this file with Avast. Result : no virus found.
Can you tell me why ?
Sincerely.
Well none AV can detect all viruses 
If you find a virus which is not detected from avast! you can send this file to virus[at]avast[dot]com in password protected archive(usually the password is “virus”), in the mail body you can write some info about the virus and the archive password
Thank you for your answer.
I’ve just sent an email with the infected file.
I hope it can help.