Malwares and worms on wordpress

I have CentOS 6 64bit server. I always found a lots of worms and malwares on hosted wordpress websites. I dont know where they are coming.

Examples of worms and malwares:

Php.Trojan.StopPost
Php.Malware.Mailbot

I run scan with ClamAV and remove the codes or the files and they always back.

Please help me, Thank you!