Also a lot of the ads that appear on these boorus have spyware or tracking cookies embedded into them (just displaying them on the screen downloads it) and some AV software, MalwareBytes for example, will tag this as malware (viruses, trojans, etc.).
I know this because my browser automatically blocks this $#!t, and the spaces these adds would have been show on my screen as such. The "honest" adds are displayed fine.