Meanwhile, it’s not just a question of how viruses act on their own: some vaccines can increase virulence. Andrew Read of Pennsylvania State University has done research with several diseases, including Marek’s disease, a common plague of chicken farms, to see how vaccination affects the virus’s evolution. He found that if a vaccine keeps the virus’s host from getting sick, but still allows the virus to persist and spread—like the poultry vaccine for H5N1 did in China—it can select for a more virulent virus.
