Changes in the type of language people use to express themselves may be a warning sign of schizophrenia but changes in attention and memory proficiency are typically the first signs.
"Such impairments can increasingly restrict professional capacity and the ability to relate," says Peter Falkai of the German Society for Psychiatry and Psychotherapy, Psychosomatics and Nerve Medicine.
However, friends, families and colleagues often only first notice these symptoms during an acute outbreak of the disease.
The symptoms of schizophrenia often develop slowly over a long period. An altered perception of reality can be another warning sign.
For those affected, sounds and colours can seem more vivid or changed.
"Some patients begin to see themselves as always the object of the actions, gestures or statements of other people," Professor Falkai said.
This can be a harbinger of hallucinations and delusions, which are typical of schizophrenia.
Timely diagnosis is important as early treatment can slow down the progression of the disease. With the right specialist medical treatment, psychosis patients can lead fairly normal lives.
The disease usually manifests itself at an early age - in men between 15 and 25 years and somewhat later in women.
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