**Abstract**

Actinomycosis is a filamentous bacterium that forms part of the normal human flora of the gastrointestinal, oropharynx and female genitalia. This indolent infection is characterized by abscess formation, widespread granulomatous disease, fibrosis, cavitary lung lesions and mass-like consolidations, simulating an active malignancy or systemic inflammatory diseases. It is subacute, chronic and variable presentation may delay diagnosis due to its capability to simulate other conditions. An accurate diagnostic timeline is relevant. Early diagnosis of pulmonary actinomycosis decreases the risk of indolent complications. Proper treatment reduces the need for invasive surgical methods. Actinomycosis can virtually involve any organ system, the infection spread without respecting anatomical variables as metastatic disease does, making malignancy an important part of the differential diagnosis. As it is normal gastrointestinal florae, it is difficult to cultivate, and share similar morphology to other organisms such as Nocardia and fungus. It is often difficult to be identified as the culprit of disease. Its true imitator capability makes this infectious agent a remarkable organism within the spectra of localized and disseminated disease. In this chapter, we will discuss different peculiarities of actinomycosis as an infectious agent, most common presentation in different organ systems, and challenging scenarios.

**Keywords:** actinomycosis, pulmonary, systemic, disseminated, imitator
