Plasmid-bearing bacteria that are resistant to a wide variety of antibiotics can be defined as relatively environmentally robust, compared to bacteria strains unable to grow in the presence of antibiotic challenges. Whether this difference arose largely through selection acting at the level of bacteria, or purely at the level of plasmids, is unclear. In the former case, acquisition and maintenance of a large number of resistance genes could be evolved "bet-hedging" in bacteria, where selected retention of these alleles occurs despite only rare circumstances when they are actually needed to protect against antibiotics. Alternatively, selection may be acting purely at the level of plasmids, favoring infectious genetic elements that collect resistance genes which may be beneficial for "paying the rent" to their bacterial hosts, as plasmids cannot easily control the host backgrounds and environments in which they reside. Last, selection may be acting on the symbiotic microbe created through plasmid/cell association, where the combined interests of plasmids and their hosts may sometimes coincide and other times conflict. Regardless, environmentally robust bacterial pathogens of humans must have been selectively favored during the past halfcentury, or the ineffectiveness of antibiotic therapy would not be a prominent issue today.