- Ichthyosis
- Idiopathic thrombocytopenia purpura (ITP, low platelets)
- Immobility (lack of movement)
- Impacted tooth
- Impetigo (bacterial skin infection)
- Impingement syndrome of shoulder (bone rubs again tendon, bursa)
- Incisional hernia
- Increased pressure in the skull (raised intracranial pressure)
- Infection
- Infectious mononucleosis (Epstein Barr virus, "Mono")
- Inflammation of testicular tubes (epididymitis)
- Inflammatory bowel disease
- Inflammatory conditions
- Influenza (seasonal flu)
- Ingrown nail (onychocryptosis)
- Inguinal hernia (groin hernia)
- Inhalants abuse (huffing)
- Inhaled foreign body (in respiratory tract)
- Insect bite
- Insomnia (difficulty sleeping)
- Insulin overdose
- Interstitial cystitis (bladder inflammation)
- Interstitial lung disease (chronic scarring of the lung)
- Intestinal ischemia (decreased intestinal blood flow)
- Intestinal volvulus (twisting of intestine)
- Intracranial hemorrhage (bleeding in brain)
- Intrahepatic cholestasis of pregnancy (liver disorder in pregnancy)
- Intraspinal abscess (collection of pus in the spine)
- Intravenous drug abuse (IVDA)
- Intussusception (telescoping intestine)
- Invasive ductal carcinoma
- Invasive lobular carcinoma
- Iritis (inflammation of the iris)
- Iron deficiency anemia (low red blood cell count)
- Iron overdose
- Irritable bowel syndrome (IBS, chronic intestinal pain and cramping)
- Itchy rash