two thirds of people with guillain–barré syndrome have experienced an infection before the onset of the condition. most commonly these are episodes of gastroenteritis or a respiratory tract infection. in many cases, the exact nature of the infection can be confirmed. approximately 30% of cases are provoked by "campylobacter jejuni" bacteria, which cause diarrhea. a further 10% are attributable to cytomegalovirus (cmv, hhv-5). despite this, only very few people with "campylobacter" or cmv infections develop guillain–barré syndrome (0.25–0.65 per 1000 and 0.6–2.2 per 1000 episodes, respectively). the strain of "campylobacter" involved may determine the risk of gbs; different forms of the bacteria have different lipopolysaccharides on their surface, and some may induce illness (see below) while others will not.