Line 47 Blues

I have a recurring problem. Every week or 10 days (give or take), a ton of code is dumped into my header.php. It’s always in Line 47. It’s always links to online pharmacy web sites, some porn, some other stuff. It’s not visible on my site, but it does cause hundreds of errors when I try to do HTML verification. I have to go into header.php and erase everything in Line 47 and then all is well again.

Any ideas how to address this?

Thanks,
Carl