Prostatitis: This is often the most common cause of blood in semen (hematospermia) due to inflammation of the prostate gland, which can lead to bleeding into the seminal fluid. Prostatitis can be acute or chronic and is usually caused by bacterial infection.

Benign Prostatic Hyperplasia (BPH): Enlargement of the prostate gland can cause bleeding into the semen due to the increased vascularity and potential for minor trauma during ejaculation.
Urethritis: Inflammation of the urethra can lead to bleeding, which may then appear in the semen.
Seminal Vesiculitis: Inflammation of the seminal vesicles, which are involved in the production of semen, can cause blood to appear in the semen.

Prostate Cancer: Although less common, prostate cancer can cause hematospermia. It is crucial to rule out this potentially life-threatening condition.
Testicular Cancer: Though rare, testicular cancer can also lead to hematospermia and must be considered to ensure timely diagnosis and treatment.
Urethral or Bladder Trauma: Any form of trauma to the urethra or bladder can cause bleeding that may appear in the semen.

Amyloidosis: A condition characterized by the deposition of amyloid (a particular type of protein) in various tissues, which can rarely cause hematospermia.
Tuberculosis: Genitourinary tuberculosis is a rare condition in many parts of the world but can cause a variety of symptoms, including hematospermia.
Vascular Malformations: Rare vascular abnormalities within the genital or urinary tract can potentially cause bleeding into the semen.