[2] The disease is characterised by the formation of painful abscesses in the mouth, lungs,[3][4] breast,[5] or gastrointestinal tract.

In severe cases, they may penetrate the surrounding bone and muscle to the skin, where they break open and leak large amounts of pus, which often contains characteristic granules filled with progeny bacteria.

[7] In animals, they normally live in the small spaces between the teeth and gums, causing infection only when they can multiply freely in anoxic environments.

An affected human often has recently had dental work, poor oral hygiene, periodontal disease, radiation therapy, or trauma (broken jaw) causing local tissue damage to the oral mucosa, all of which predispose the person to developing actinomycosis.

[8] They are also normal commensals among the gut flora of the caecum; thus, abdominal actinomycosis can occur following removal of the appendix.

[13] In 1877, pathologist Otto Bollinger described the presence of A. bovis in cattle, and shortly afterwards, James Israel discovered A. israelii in humans.

After Bostroem's discovery, a general misconception existed that actinomycosis was a mycosis that affected individuals who chewed grass or straw.

Actinomycosis occurs rarely in humans, but rather frequently in cattle as a disease called "lumpy jaw".