WordPress Virus?!?!

I was downloading a backup of one of my sites when my security program told me i had a virus, the file was in my public_html folder it was wp-ls.php I am running Microsoft Security Essentials it called the virus “Backdoor:PHP/C99shell.H”
I am really good with wordpress installing configuring and im the same with web and database servers and even i don’t know whats going on but i do know that its malicious code.

I tried to post it on pastebin but it didn’t work it exceeded 500 kilobytes and i dont think i can upload it

Whats going on?!?