This category lists works of (usually horror-based) speculative fiction (fantasy, science fiction, or science fantasy) that revolve around the rapid mass spread of a fictional infectious disease which transforms people into mutated and/or undead monsters. These creatures are considered to be no longer human anymore in both body and mind, often being hostile and predatory towards uninfected people.

While most of these stories feature an apocalyptic pandemic that results in the catastrophic collapse of global human civilization, this category also includes works involving a more regional epidemic, or a small-scale outbreak confined to a much smaller location which does not destroy or disrupt society as a whole.

Please see the subcategories Category:Fiction about vampire outbreaks and Category:Fiction about zombie outbreaks for works about viral outbreaks of vampirism and zombification respectively. Pages placed directly into this category should involve monsters that are not easily categorized as vampires nor zombies.