The cause of this disease has not yet been identified.
When pathogenic bacteria enter the body, they cause infectious diseases.
The doctors finally identified the cause of the mysterious illness.
Contaminated water can be the cause of diarrhea.
病原体 specifically names the causative organism (bacteria, virus, etc.), whereas 病原 can refer more abstractly to any cause of illness.
Standard kanji spelling; used in medical and general contexts.
Variant spelling; less common than 病原.
Compound of 病 (disease) and 原 (origin, source). The spelling 病源 uses 源 (source) with a similar meaning, but 病原 is the standard form.