According to the Mayo Clinic, aphasia typically occurs suddenly, after a stroke or a head injury, and can affect a person’s ability to speak, write and understand language, both verbal and written. The condition can also gradually occur “from a slow-growing brain tumor or a disease that causes progressive, permanent damage.”
Article source: https://www.huffpost.com/entry/bruce-willis-retiring-aphasia-diagnosis_n_62448631e4b0587dee641d60