He describes other aspects of gay sexual ecology that epidemiologists have cited for this rapid spread of HIV in addition to high levels of partners and highly active core groups. Among them are insertive/receptive ‘versatility, the practice of ‘concurrency,’ or having multiple partners concurrently as opposed to one at a time as in serial multipartnerism, the impact of viral load, the synergistic impact of multiple re-infections with various STDs which can lower immunity, the impact of substance abuse which can also lower immunity, and the ability of travel to move infections rapidly around the country. He also cites the theory of evolutionary biologist Paul W. Ewald that high levels of HIV transmission among gay men led to the evolution of more virulent strains.