In their environment, plants establish relationships with many microorganisms such as fungi, bacteria and viruses which can be either pathogens or symbionts. Common wisdom states that mutualistic/symbiotic associations are beneficial to the plant but pathogen associations deleterious. However, recent papers in New Phytologist by Xu et al. (this issue; pp. 911–921) and Baltruschat et al. (2008; issue 180 (2); pp. 501–510) underline the point that this view is perhaps too simplistic. In the case of pathogenesis, one possibility for the plant to prevent or minimize microbe infection is to generate an oxidative burst, the purpose of which is to kill bacteria and plant cells surrounding the infection site.