Cross-site contamination happens when a hacked site infects other sites on a shared server. Think of it as your kid in daycare catching the flu, next thing you know, everybody in the family has it as well. The same happens with websites. A site can be negatively affected by neighboring sites that are on the same server.
One of the main causes of cross-site contamination is poor isolation on the server or weak account configuration.
Source: Scuri check