Depends on the host as another site may be infected with malware on the very server your pages are served from. Then you have some services intentionally inject their own code in as part of their service terms.
So check the web host you are using has not got a problem with infection. If you keep removing it and they reappear, then its very likely that the issue is on the server not external.
If it is external then the host needs to up their game a bit in terms of all latest security updates.
Otherwise, find a reputable hosting company.