Final answer:
Naegleria fowleri is the causative agent of primary amoebic meningoencephalitis, a rare and usually fatal brain infection that occurs when contaminated water is inhaled through the nose.
Step-by-step explanation:
The causative agent of primary amoebic meningoencephalitis (PAM) is Naegleria fowleri. This amoeboflagellate is typically found in freshwater environments, such as lakes and rivers. Infection occurs when water containing the amoeba enters the body through the nasal passages. N. fowleri then travels to the brain, causing severe inflammation and destruction of brain tissue, leading to symptoms like headaches, fever, confusion, and seizures. Although rare, PAM is a rapid and nearly always fatal disease.