Merck Manual

Please confirm that you are a health care professional

honeypot link

Manifestations of Infection


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Reviewed/Revised Aug 2022 | Modified Sep 2022


Most infections increase the pulse rate and body temperature, but others (eg, typhoid fever, tularemia, brucellosis, dengue) may not elevate the pulse rate commensurate with the degree of fever (relative bradycardia). Hypotension can result from hypovolemia, septic shock, or toxic shock Toxic Shock Syndrome (TSS) Toxic shock syndrome is caused by staphylococcal or streptococcal exotoxins. Manifestations include high fever, hypotension, diffuse erythematous rash, and multiple organ dysfunction, which... read more Toxic Shock Syndrome (TSS) . Hyperventilation and respiratory alkalosis are common.

Alterations in sensorium (encephalopathy) may occur in severe infection regardless of whether central nervous system infection is present. Encephalopathy is most common and serious in older adults and may cause anxiety, confusion, delirium, stupor, seizures, and coma.


Infectious diseases commonly increase the numbers of mature and immature circulating neutrophils. Mechanisms include demargination and release of immature granulocytes from bone marrow, interleukin-1– and interleukin-6–mediated release of neutrophils from bone marrow, and colony-stimulating factors elaborated by macrophages, lymphocytes, and other tissues. Exaggeration of these phenomena (eg, in trauma, inflammation, and similar stresses) can result in release of excessive numbers of immature leukocytes into the circulation (leukemoid reaction), with leukocyte counts up to 25,000 to 30,000/mcL (25 to 30 × 109/L).

Conversely, some infections (eg, typhoid fever, brucellosis) commonly cause leukopenia. In overwhelming, severe infections, profound leukopenia is often a poor prognostic sign.

Characteristic morphologic changes in the neutrophils of septic patients include Döhle bodies, toxic granulations, and vacuolization.

Anemia can develop despite adequate tissue iron stores. If anemia is chronic, it is a normochromic, normocytic anemia characterized by low serum iron, low total iron-binding capacity, and normal to increased serum ferritin.

Other organ systems

Hepatic dysfunction, including cholestatic jaundice (often a poor prognostic sign) or hepatocellular dysfunction, occurs with many infections, even though the infection does not localize to the liver.

Gastrointestinal (GI) manifestations include upper GI bleeding due to stress ulceration that may occur during sepsis.

Endocrinologic dysfunctions include

  • Increased production of thyroid-stimulating hormone, vasopressin, insulin, and glucagon

  • Breakdown of skeletal muscle proteins and muscle wasting secondary to increased metabolic demands

  • Bone demineralization

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!