It is unknown why some individuals become infected with N. fowleri and others do not. Both dose and strain of N. fowleri may be factors. Also playing a role in susceptibility may be defects in the infected person’s innate or acquired immunity. It is postulated that the amoeba enters the nose, invades the nasal mucosa, crosses the cribriform plate, and ultimately reaches the olfactory bulb. From there, it penetrates the cortex and periventricular system where it incites meningoencephalitis, resulting in rapid cerebral edema and cerebellar herniation. Symptoms are usually evident in one to seven days after exposure, but it could be as long as 12 days. The signs and symptoms of the infection are similar to those of bacterial or viral meningitis.