E-Mail Developing vaccines against bacteria is in many cases much more difficult than vaccines against viruses. Like virtually all pathogens, bacteria are able to sidestep a vaccine's effectiveness by modifying their genes. For many pathogens, such genetic adaptations under selective pressure from vaccination will cause their virulence or fitness to decrease. This lets the pathogens escape the effects of vaccination, but at the price of becoming less transmissible or causing less damage. Some pathogens, however, including many bacteria, are extremely good at changing in ways that allow them to escape the effects of vaccination while remaining highly infectious. For scientists looking to develop vaccines, this kind of immune evasion has been a fundamental problem for decades. If they set out to develop vaccines against bacterial pathogens, often they will notice that these quickly become ineffective.