M. catarrhalis is a human Gram-negative pathogen with an affinity for the human upper respiratory tract. Other primates, such as macaques, might become infected by this bacterium.
In humans, it can cause infections of the respiratory system, middle ear, eye, central nervous system, and joints.
M. catarrhalis has recently been gaining attention as an emerging human pathogen. It has been identified as an important cause in bronchopulmonary infection, causing infection through pulmonary aspiration in the upper pulmonary tract. Additionally, it causes bacterial pneumonia, especially in adults with a compromised immune system. It has also been known to cause infective exacerbations in adults with chronic lung disease, and it is an important cause in acute sinusitis, maxillary sinusitis, bacteremia, meningitis, conjunctivitis, acute purulent irritation of chronic bronchitis, urethritis, sepsis (although this is rare), septic arthritis (which is also a rare occurrence),and acute laryngitis in adults and acute otitis media in children. M. catarrhalis is an opportunistic pulmonary invader, and causes harm especially in patients who have compromised immune systems or any underlying chronic disease. M. catarrhalis has also been linked with septic arthritis in conjunction with bacteremia.