Systemic infection is a generic term for infection caused by microorganisms in animals or plants, where the causal agent (the microbe) has spread actively or passively in the host's anatomy and is disseminated throughout several organs in different systems of the host. In the case of animals, throughout organs in the digestive, respiratory, and other systems, especially the circulatory system; in plants, throughout the xylem and/or phloem vessels, and into organs like leaves, stems, roots, tubers, fruits, et cetera.

Systemic infections are also called disseminated infections, and they can be caused by bacteria and bacteria-like prokaryotes, fungi, protozoa in the broad sense, and viruses.