Infections caused by exposure to ionizing radiation can be extremely dangerous, and are of public and government concern. Numerous studies have demonstrated that the susceptibility of organisms to systemic infection increased following exposure to ionizing radiation. The risk of systemic infection is higher when the organism has a combined injury, such as a conventional blast, thermal burn, or radiation burn. There is a direct quantitative relationship between the magnitude of the neutropenia that develops after exposure to radiation and the increased risk of developing infection. Because no controlled studies of therapeutic intervention in humans are available, almost all of the current information is based on animal research. Cause of infection Infections caused by ionizing radiation can be endogenous, originating from the oral and gastrointestinal bacterial flora, and exogenous, originating from breached skin following trauma. The organisms causing endogenous infections are generally gram negative bacilli such as Enterobacteriaceae (i.e. Escherichia coli, Klebsiella pneumoniae, Proteus spp. ), and Pseudomonas aeruginosa. Exposure to higher doses of radiation is associated with systemic anaerobic infections due to gram negative bacilli and gram positive cocci. Fungal infections can also emerge in those that fail antimicrobial therapy and stay febrile for over 7–10 days. Exogenous infections can be caused by organisms that colonize the skin such as Staphylococcus aureus or Streptococcus spp. and organisms that are acquired from the environment such as Pseudomonas spp. Organisms causing sepsis following exposure to ionizing radiation: Principles of treatment The management of established or suspected infection following exposure to radiation (characterized by neutropenia and fever) is similar to that used for other febrile neutropenic patients. However, important differences between the two conditions exist. The patient that develops neutropenia after radiation is susceptible to irradiation damage to other tissues, such as the gastrointestinal tract, lungs and the central nervous system. These patients may require therapeutic interventions not needed in other types of neutropenic infections. The response of irradiated animals to antimicrobial therapy is sometimes unpredictable, as was evident in experimental studies where metronidazole and pefloxacin therapies were detrimental. Antimicrobial agents that decrease the number of the strict anaerobic component of the gut flora (i.e., metronidazole) generally should not be given because they may enhance systemic infection by aerobic or facultative bacteria, thus facilitating mortality after irradiation. Choice of antimicrobials An empirical regimen of antibiotics should be selected, based on the pattern of bacterial susceptibility and nosocomial infections in the particular area and