- Sagittal sinus thrombosis (venous blood clot in the brain)
- Salmon patch (common birthmark)
- Salmonella infection (common bacterial intestinal infection)
- Sandhoff disease
- Sarcoidosis (inflammatory disease)
- Sarcoma (soft tissue cancer)
- Scabies (skin disease caused by mites)
- Scalded skin syndrome (blistering skin infection)
- Scalp laceration (scalp cut)
- Scalp swelling
- Scaphoid fracture (broken hand bone)
- Scar
- Scarlet fever (post strep inflammation)
- Schistosomiasis (parasite infection)
- Schizoaffective disorder (features of schizophrenia and mood disorder)
- Schizophrenia (chronic impaired reality perception)
- Sciatica (lower back nerve irritation)
- Scleritis (inflammation of the eye)
- Scleroderma (progressive hardening of soft tissue)
- Sclerosing cholangitis (progressive bile duct disease )
- Scoliosis (crooked spine)
- Scorpion sting
- Scrotal hematoma (collection of blood in skin around testicles)
- Scurvy (vitamin C deficiency)
- Sea lice
- Seasonal affective disorder (intermittent seasonal depression)
- Sebaceous cyst (sac under the skin)
- Seborrheic dermatitis
- Seborrheic keratosis
- Secondary dysmenorrhea (painful menstruation)
- Seizure (abnormal brain electrical activity)
- Self-inflicted injury
- Sensitive teeth
- Sepsis (serious blood infection)
- Septal perforation (hole in the skin between nostrils)
- Septic arthritis (joint infection)
- Septic hip (infected hip joint)
- Septic knee joint (infected knee joint)
- Septic shock (low blood pressure from serious blood infection)
- Seroma (fluid build-up)
- Serotonin syndrome (severe medication reaction)
- Serum sickness (abnormal immune system response)
- Sever's disease (calcaneal apophysitis, common cause of heel pain)
- Severe Acute Respiratory Syndrome (SARS, serious viral pneumonia)
- Sexually transmitted disease (STD)
- Shigellosis (bacterial intestinal infection)
- Shin splints (tibial stress syndrome)
- Shingles (herpes zoster)
- Shock (severe low blood pressure)
- Shoulder joint dislocation (dislocated shoulder)
- Shoulder sprain
- Sialadenitis (salivary gland infection)
- Sialoliths (salivary gland stones)
- Sickle cell crisis (complication of sickle cell disease)
- Sickle cell disease (disease of red blood cells)
- Sinusitis (sinus infection)
- Situational depression
- Sjogren's syndrome (dry eyes and mouth)
- Skin abscess (collection of pus)
- Skin blood vessel damage (Chilblains, pernio)
- Skin laceration (cut skin)
- Skin swelling
- Skin tags (acrochordons)
- Skin ulcer
- Skull fracture (broken bone)
- Sleep apnea (chronic sleep disruption)
- Slipped capital femoral epiphysis (adolescent hip disorder)
- Slurred speech (dysarthria)
- Small bowel cancer (small intestine)
- Small bowel lymphoma (intestinal lymph node cancer)
- Small cell lung cancer
- Smallpox (Variola virus infection)
- Smoke inhalation
- Smoking (tobacco use disorder)
- Social anxiety disorder (social phobia)
- Somatization disorder (Somatic symptom disorder)
- Spermatocele (sperm-filled epididymal cyst)
- Spider veins
- Stroke (CVA, cerebrovascular accident)
- Stuttering
- Spina bifida
- Spinal cord compression (cauda equina syndrome)
- Spinal abscess (collection of spinal pus)
- Spinal cord injury, acute
- Spinal cord tumor
- Spleen injury (splenic rupture)
- Splenic infarction (death of spleen)
- Sprain (stretched, torn ligament)
- Spondylolisthesis (slipped back bone)
- Squamous cell lung cancer
- Squamous cell cancer (common skin cancer)
- Staphylococcus aureus (type of bacteria)
- Steroid use
- Steroid abuse
- Stomach cancer (gastric cancer)
- Steven Johnson syndrome (rare, serious skin disorder)
- Stomatitis, denture-induced
- Strabismus (crossed eyes)
- Strawberry hemangioma (blood vessel birthmark)
- Strangulation of genitalia (constriction of genital blood supply)
- Streptococcus infection (strep throat bacteria)
- Stress
- Stress headache (tension headache)
- Stress incontinence (unintentional urination with cough or sneeze)
- Subarachnoid hemorrhage (brain bleeding)
- Subconjunctival hemorrhage (blood covering white part of the eye)
- Subdural hematoma (bleeding on brain)
- Subluxation
- Substance abuse
- Subungual hematoma (blood under nail)
- Sudden Cardiac Death (SCD)
- Sudden Infant Death Syndrome (SIDS)
- Suicide
- Sun poisoning
- Sunburn
- Supracondylar fracture (elbow fracture)
- Supraventricular tachycardia (SVT)
- Swimmer's itch (cercarial dermatitis)
- Swine Flu (H1N1 Influenza)
- Symptomatic dermatographism (exaggerated skin response to pressure)
- Synovitis (inflammation of joint lining)
- Syphilis (treponema pallidum)
- Systemic sclerosis (scleroderma, progressive soft tissue hardening)