- Rabies (deadly viral illness transmitted by animals)
- Radial head fracture (broken elbow bone)
- Radiation exposure (radiation sickness)
- Radiator fluid (ethylene glycol) poisoning
- Rattlesnake (pit viper) bite
- Ramsay Hunt syndrome (viral infection causing facial weakness)
- Rash
- Raynaud's disease (blood vessel constriction)
- Rectal abscess (collection of pus in the rectum)
- Rectal ulcer
- Reflex sympathetic dystrophy (complex pain syndrome)
- Rectovaginal fistula (internal rectum to vagina connection)
- Rectocele
- Reiter syndrome (type of joint inflammation)
- Renal artery stenosis (kidney artery narrowing)
- Renal arteriovenous malformation (AVM, blood vessel abnormality of the kidney)
- Renal failure, acute (sudden kidney failure)
- Renal failure, chronic (ongoing kidney failure)
- Renal infarct (kidney damage from an area of blocked blood flow)
- Renal trauma (kidney trauma)
- Renal vein thrombosis (kidney vein clot)
- Respiratory failure (breathing failure, ARDS)
- Restless legs syndrome (nervous system disorder)
- Respiratory syncytial virus (RSV, pediatric respiratory infection)
- Retinal vein occlusion
- Retinal artery occlusion
- Retinitis pigmentosa
- Retinoblastoma (pediatric eye cancer)
- Retinopathy
- Retrobulbar Neuritis
- Retroperitoneal hemorrhage (flank bleeding)
- Retropharyngeal abscess (throat abscess)
- Reyes syndrome (rare liver and brain swelling)
- Rhabdomyolysis (severe muscle breakdown)
- Rheumatic fever (complication of strep infection)
- Rheumatoid arthritis (type of chronic joint inflammation)
- Rhinitis (nasal inflammation)
- Rhinosporidiosis (fungal nasal infection)
- Rhinovirus infection (common cold)
- Rib fracture (broken bone)
- Rickets (childhood bone disease)
- Rickettsial infections (type of bacterial infection)
- Ringworm of the body (tinea corporis)
- Ringworm of the scalp (tinea capitis)
- Rocky mountain spotted fever (bacterial infection spread by ticks)
- Rosacea
- Rotator cuff syndrome (shoulder injury)
- Rotavirus infection (intestinal viral infection)
- Rubbing (isopropyl) alcohol overdose
- Rubella (German measles)