A method and apparatus for economically filtering air into swine farms and other animal enclosures are provided. Advantageously, the invention substantially reduces the risk to airborne transfer of disease into animal enclosures. In one embodiment, a method for providing clean air to an animal enclosure includes providing an animal enclosure having an animal containing volume for containing a plurality of animals, removing air from the internal volume via at least one exhaust fan, and filtering air being pulled into the enclosure by the at least one exhaust fan using a fully sealed filter, the fully sealed filter having a particle separation sufficiently efficient to prevent detection of live (PRRS-) virus downstream of the filter, using polymerase chain reaction (PCR) analysis of samples collected with a cyclonic aerosol collector.