At the age of 16, she was diagnosed with epilepsy after suffering a severe convulsion. The first signs of demonic possession occurred in 1973 when she began to suffer from depression and started hearing voices which were telling her that she was "damned" and that she would "rot in hell".
She later began treatment in an unnamed psychiatric hospital. However this treatment did not improve her health. Instead, her depression worsened. She had grown increasingly frustrated and annoyed with any sort of medical intervention as it didn't help. All types of long-term treatment proved unsuccessful, and her conditions worsened in time.
Anneliese began to contrast her condition to demonic possession. She became intolerant of sacred objects and places, such as the crucifix, as she attributed this to her own demonic possession.
In June 1970, she suffered a 3rd seizure at the psychiatric hospital she had been staying, and then was prescribed anti-convulsants for the first time. She talked about "devil faces", which she claimed to see at various times during the day. She was increasingly adamant that her illness was of a spiritual kind, and was convinced that no medicine could help her. She took several different types of drugs up until shortly before her death.
On a pilgrimage to San Damiano with a good friend of the family, Anneliese managed to get an exorcism. After some examining from a vicar, who after seeing Anneliese, declared that she didn't "look like an epileptic" and that he didn't see her having seizures. Also she refused to walk pas a crucifix and refused to drink the water of a holy spring. The vicar believed she was suffering from demonic possession, and he urged the bishop to allow an exorcism.
After being convinced that she was possessed, Anneliese and her parents stopped seeking medical treatment, and decided to solely put her fate into the hands of the exorcism rites.
Over 10 months, between 1975 and 1976, 67 sessions of exorcism, one or two each week, lasting for up to 4 hours, were performed on Anneliese. At some point, she began talking about dying to atone for the wayward youth of the day, and she refused to eat.
On 1 July 1976, Anneliese died in her home. An autopsy reported the cause of death as malnutrition and dehydration.
Below is a short clip of the exorcism:
Below is the full audio tapes for the exorcism:
Several movies have been loosely based on this case, such as The Exorcism of Emily Rose