WordPress Website Keep Getting Malicious Code Injected

[ Moderator note: moved to Fixing WordPress. ]

I have wordpress sites that keep getting malicious code injected into the pages. How can I stop them from happening as I have to keep going into them and removing the line.

I have updated wordpress and try some malware scanner but codes like “<script src=’URL’ type=’text/javascript’></script>” keeps getting injected causing site redirects. I can remove it but it annoying having to do this every time.