Cross-site contamination occurs when a clean website is negatively affected by another infected site.
Infected sites might be active and maintained or simply abandoned or forgotten – we call them ghost sites since these are live but no longer maintained.
Cross-site contamination risk appears when you keep at least two sites in the same hosting space.
The more sites are added, the greater the risks.
From our experience, we noticed that WordPress sites are prone to generate the primary contamination.
Hosting environments are often cPanel based.
So the recipe for a cross-site contamination disaster often consist in having few outdated sites ( WordPress, Joomla, Drupal, etc ) hosted on a single shared hosting plan ( cPanel ).
In this case, due to poor isolation, malware spreads easily.
Relevant articles:
https://blog.sucuri.net/2017/07/cross-site-contamination-and-how-to-prevent-it.html