In one experiment, unvaccinated birds infected with the most virulent strains were housed together with healthy birds. Again, the infected chickens were dead in no time, leaving them no chance to spread the disease to their healthy cagemates. But when vaccinated birds were infected with the highly virulent strain, they lived longer and all the healthy birds housed with them became infected and died. Thus, "vaccination enabled the onward transmission of viruses otherwise too lethal to transmit, putting unvaccinated individuals at great risk of severe disease and death,” the authors write online today in PLOS Biology.