Predictably, dramatic changes were observed in the blood of patients with systemic infections (for example, sepsis) [76, 77] . However, profound alterations in patterns of transcript abundance were also found in patients with localized infections (for example, upper respiratory tract infection, urinary tract infections, pulmonary tubercu lo sis, skin abscesses) [13, 16, 78] . Measuring changes in host transcriptional profi les may therefore prove of diagnostic value even in situations where the causative pathogenic agent is not present in the test sample. Importantly, it may also help ascertain the severity of the infection and monitor its course.