At the 1990s' emergence of epidemiological associations between dental infections and systemic diseases, American dentistry scholars have been cautious, some seeking successful intervention to confirm causality. Some American sources emphasized epidemiology's inability to determine causality, categorized the phenomena as progressive invasion of local tissues, and distinguished that from focal infection theory—which they assert was evaluated and disproved by the 1940s. Others have found focal infection theory's scientific evidence still slim, but have conceded that evolving science might establish it. Yet select American authors affirm the return of a modest theory of focal infection.