This study describes 5th, 8th, and 11th-grade students', teachers', and medical professionals' conceptions of flu and microbial illness. Participants constructed a concept map on "flu" and participated in a semi-structured interview. The results showed that these groups of students, teachers and medical professionals held and structured their conceptions about microbes differently. A progression toward more accurate and complete knowledge existed across the groups but this trajectory was not always a predictable, linear developmental path from novice to expert. Across the groups, participants were most knowledgeable about symptoms of microbial illness, treatments of symptoms, and routes of transmission for respiratory illnesses. This knowledge was tightly linked to participants' prior experiences with colds and flu. There were typically large gaps in participants' (children and teachers) understandings of vaccines, immune system responses, treatments (including the mechanisms of pain medications and the functions of antibiotics), and transmission of non-respiratory microbial illness. A common misconception held by students was the belief that antibiotics can cure viral infections.