I'll quickly start with a little pathology to define what we're talking about. Bacteria are little single celled organisms, they're everywhere, and many of them are good. It's when they start dividing too much and producing toxins do they cause us disease. The virus is a very very small (even compared to bacteria) package of protein and either DNA or RNA. Really, the virus is just some genetic code in a box. The way the immune system works is for the sake of this question, a 3 part process. First it recognizes the intruder, then it kills it off, then it remembers it for quicker killing next time. Vaccination is the process by which the body is exposed to either a dead or disabled version of the pathogen. By doing so, the body is able to recognize the intruder, and learn what it is without having a huge battle. This way, were your body to meet the intruder again, it's quick to kill it before it can cause an infection. Comparatively speaking, it's far easier to produce a viral vaccine than a bacterial. A virus is easier to disable to present to the body. When this is tried with bacteria, it's found that the dead bacteria "look" different to the body than the live ones. Bacteria, being live (viruses are arguably not alive) organisms can react and change their appearances once they enter the body, so the vaccine is likely to not be effective. Furthermore, the focus on viral vaccines has been far stronger in the history of medicine because of our lack of treatment for viral infections. Until recent years, no drugs existed to fight viruses once they invaded the body - we just had to hope the immune system could fight them off. We've had antibiotics for some time... so the logical trend was to use preemptive medicine against the infections we couldn't treat. Hope this helps! --SP