@hellehelle When you enable the plugin, you have to read the description and understand who is it for and what it does.
The plugin will capture the final HTML and during that time, it will merge all CSS and JS files that you added on the settings. It will then link to those new cache files and the final page will be generated.
If you have malware already on your css or js files, FVM will simply merge whatever you already have there. However, when the optimization is enabled, you now will see the merged malware on the cache files under the FVM cache directory.
This is obvious, because you are no longer accessing the original files but rather, a cache file where the original files were copied to.
If you clear the cache, FVM will repeat the process. It will again read your original files, minify and combine them into a new cache file.
When you uninstall the plugin… the merging and minification of css and js files stops and those cache files are deleted. The site will then revert to the original files, as enqueued by your theme and other plugins.
The only time you are going to see 404 errors on FVM cache files, is when your server is still having page cache in memory.
If your full page was cached on your hosting provider when FVM was active, it would be having css and js references to cache files that are now deleted. The issue there, is that you haven’t correctly purged your full page cache, be it some cache plugin, hosting cache or whatever… so the cached version, is now pointing to expired and deleted cache files.
This is explained on the HELP section when you install the FVM plugin.
Some hosting services have persistent caches for disk or page cache.
This means, if you delete (or add) a plugin, it may not work until your hosting clears the cache on the server and it fetches again the actual files from the remote storage.
Not many hosting services do this, so I recommend you to switch to some other provider, like godaddy, bluehost, cloudways, or whatever.
It’s nothing to do with being like a virus. If you are deleting the plugin, and if you don’t see it on wp-admin anymore, that means it’s gone. It’s your server that is caching too much, and still has that code in memory.
It can happen with any other plugin that does css and js minification as well.
You just think it’s the plugin fault, because you don’t understand what your server is doing and how it keeps code in memory, and how you need to clear all caches properly whenever you install or deactivate a plugin.
Another possibility may be file permissions. If your server is not configured correctly, you can ask your hosting to fix those permissions.
Likewise, if you see 404 errors on expired FVM css and js files even after you have removed the plugin (and the plugin is gone from wp-admin), you need to ask your hosting why is that. There is likely something wrong there with OPCache, which is not clearing when it needs to, or you did not clear the page caching.
I would suggest you to contact your hosting or hire a developer to help you with that, as speed optimization, as the plugin description says, is for developers and advanced users. And your server also needs to be compatible, so a developer will check that for you as well.
