Version July 2025
| Microbial agent | Susceptible hosts |
| Bacteria | |
| Haemophilus influenzae (typeable and nontypeable) | Unimmunized (H. influenzae type b) |
| Moraxella catarrhalis | |
| Neisseria meningitidis (often group Y) | |
| Klebsiella spp | Immunocompromised hosts, nosocomial pathogen |
| Escherichia coli | Immunocompromised hosts, nosocomial pathogen |
| Enterobacter spp | Immunocompromised hosts, nosocomial pathogen |
| Pseudomonas aeruginosa | Immunocompromised hosts, nosocomial pathogen, cystic fibrosis |
| Burkholderia cepacia | Cystic fibrosis patients |
| Achromobacter xylosoxidans | Cystic fibrosis patients, nosocomial pathogen |
| Stenotrophomonas maltophilia | Cystic fibrosis patients, nosocomial pathogen |
| Legionella pneumophila | Immunocompromised hosts |
| Pseudomonas pseudomallei | Travelers to or residents of endemic areas |
| Francisella tularensis | Exposure to a particular animal reservoir (rabbits) or insect vector; bioterrorist activity |
| Brucella abortus | Exposure to a particular animal reservoir (cattle, goats) |
| Chlamydia psittaci | Exposure to a particular animal reservoir (psittacines – parrots, parakeets, macaws, and virtually all domestic and wild birds) |
| Leptospira spp | Exposure to a particular animal reservoir |
| Yersinia pestis | Exposure to a particular animal reservoir or insect vector (rats); bioterrorist activity |
| Anaerobic mouth flora (Prevotella spp, Fusobacterium, Bacteroides spp) | Aspiration |
| Mycobacterium tuberculosis | Travelers to or residents of endemic areas, contact with known TB, homeless, recent immigrants from endemic areas, prisoners, and HIV-infected individuals |
| Nontuberculous mycobacterium | Cystic fibrosis patients, immunocompromised hosts |
| Bordetella pertussis | Infants, exposure to adult with a cough illness |
| Fungi | |
| Coccidioides immitis | Travelers to or residents of endemic areas (southwest United States) |
| Histoplasma capsulatum | Travelers to or residents of endemic areas (Ohio, Mississippi, and Missouri River valleys); also occurs in Canada, Central America, eastern and southern Europe, parts of Africa, eastern Asia, and Australia |
| Blastomyces dermatitidis | Travelers to or residents of endemic areas (southeastern and central United States and midwestern states bordering the Great Lakes) |
| Candida spp | Nosocomial pathogen, immunocompromised hosts, cystic fibrosis |
| Aspergillus spp | Nosocomial pathogen, immunocompromised hosts, cystic fibrosis |
| Pneumocystis jiroveci (formerly carinii) | Immunocompromised hosts |
| Rickettsiae | |
| Coxiella burnetii | Exposure to a particular animal reservoir (sheep) |
| Rickettsia rickettsii | Exposure to a particular insect vector |
| Viruses | |
| Rubeola | Travelers to or residents of endemic areas |
| Herpes simplex virus | Neonates and immunocompromised hosts |
| Cytomegalovirus | Immunocompromised hosts, first three months after birth |
| Epstein-Barr virus | Immunocompromised hosts |
| Varicella zoster virus | Immunocompromised hosts |
| Coronavirus (SARS-CoV, MERS-CoV, SARS-CoV-2) | |
| Enterovirus (D68) | Primarily children <5 years, adolescents, and children with asthma |
| Rubella | |
| Hantavirus | Travelers to or residents of endemic areas, exposure to a particular animal reservoir (mouse droppings) |
| HIV | |
| Mumps | |
| Human bocavirus | Primarily children <5 years |
| Parechovirus | Primarily children <5 years |
| Avian influenza | Exposure to birds; travel to affected area |
