- Macular degeneration (chronic eye disease)
- Magic mushroom ingestion (psilocybin)
- Major depressive disorder (severe depression)
- Malabsorption syndrome (abnormal intestinal digestion)
- Malaria (mosquito transmitted illness)
- Male infertility
- Male pattern hair loss (hereditary-patterned baldness)
- Malignant hyperthermia (rare temperature abnormality)
- Malignant Mesothelioma
- Mallet finger (finger tendon rupture)
- Mallory Weiss syndrome (esophageal tear)
- Malnutrition
- Marfan syndrome (rare inherited disease)
- Marijuana use
- Mastitis (breast infection inflammation)
- Mastocytosis (urticaria pigmentosa, skin disease)
- Mastoiditis
- Maxillary sinus carcinoma (sinus cancer)
- Mayer Rokitansky Küster Hauser syndrome (MRKH, inherited vaginal abnormality)
- Measles (viral disease with typical rash)
- Meckel's diverticulum (bulge in small intestine)
- Meconium aspiration (newborn respiratory problem)
- Mediastinitis (inner chest inflammation)
- Medication reaction
- Melanoma (skin cancer)
- Melasma (choloasma)
- Meniere's disease (dizziness and vertigo)
- Meningioma (brain tumor)
- Meningitis (spinal fluid infection)
- Meniscus injury (knee cartilage injury)
- Menopause (end of menstruation and fertility)
- Meralgia Paresthetica
- Mesenteric adenitis (intestinal lymph node enlargement)
- Methamphetamine abuse
- Methemoglobinemia (blood disorder)
- Metastatic neoplasm (cancer spread)
- Methicillin-resistant Staphylococcus aureus (MRSA, type of bacteria)
- Methyl alcohol intoxication (methanol)
- Micturition syncope (fainting with urination)
- Middle East Respiratory Syndrome (MERS)
- Migraine (type of headache)
- Miller Fisher Syndrome (rare nerve disease)
- Miscarriage
- Missed abortion
- Mitral regurgitation (leaking heart valve)
- Mitral stenosis (tight heart valve)
- Mitral valve prolapse (abnormal heart valve)
- Mittelschmerz (ovulation pain)
- Molar pregnancy (gestational trophoblastic disease, pregnancy-related tumor)
- Moles (nevi)
- Molluscum contagiosum
- Mongolian spots (blue birthmark)
- Mononucleosis (common viral illness)
- Morton's neuroma
- Mosquito bite
- Motion sickness
- Motor neuron disease (progressive nerve disorder)
- Mouth cancer (oropharyngeal cancer)
- Mouth laceration (mouth cut)
- Mouth swelling
- Mouth trauma (injury)
- Mucocele
- Mucormycosis (fungal infection)
- Mucocutaneous leishmaniasis (parasitic infection)
- Multiple myeloma (blood cancer)
- Multiple sclerosis (MS)
- Mumps (viral infection involving salivary glands)
- Multiple pregnancy (twins, triplets, etc)
- Munchausen syndrome (fabricated illness)
- Muscle and ligament inflammation (enthesopathy)
- Muscle spasm (charley horse)
- Munchausen syndrome by proxy (fabricated illness in a dependent)
- Muscle strain (pulled muscle)
- Muscle tear
- Myasthenia gravis (autoimmune disease of the nerves, muscles)
- Muscular dystrophy (inherited muscle disease)
- Mycoplasma genitalium (sexually transmitted disease)
- Mycoplasma infection (bacteria)
- Myocardial infarction (heart attack)
- Myelodysplastic syndrome (bone marrow failure)
- Myocarditis (heart muscle inflammation)
- Myopathy (muscle disease)
- Myositis (muscle inflammation or infection)
- Myotonia (impaired muscle relaxation)
- Myringitis bullosa (ear drum infection)
- Myxedema coma (severely low thyroid hormone)