How to overcome a virus on websites?

Question

Malware

There is a hosting with 4 sites. Two sites on wordpress, and two on html (on one of them there is js, and the other is pure html).
2 sites were hacked (1-wp, the other is pure html)
The essence of the hack:
HTML: index.php is created that generates a page with the goods of some Japanese
Wordpress store: the same as the previous case, but the virus is written into an already existing index.php .
After removal after 30-60 minutes. the virus is back.
folders appeared on the hosting and I can't delete the TMP/run/httpd.pid (6 bytes) file, besides this, files with some marks and dates appear on all 4 sites.

The question is, where can the root of this infection sit? While completely changing all the files of the engine, this crap appears again.

Answer 1

1. Scan AI-Bolit
2. Remove viruses
3. Update website engines

Answer 2

through filezilla can be a hack. that is, the root of the infection is in the ftp client