(M)
Malignant sore throat - Diphtheria
Mania - Insanity
Marasmus - Progressive wasting away of body, like malnutrition
Membranous Croup - Diphtheria
Meningitis - Inflations of brain or spinal cord
Metritis - Inflammation of uterus or purulent vaginal discharge
Miasma - Poisonous vapors thought to infect the air
Milk fever - Disease from drinking contaminated milk, like undulant fever or brucellosis
Milk leg - Post partum thrombophlebitis
Milk sickness - Disease from milk of cattle which had eaten poisonous weeds
Mormal - Gangrene
Morphew - Scurvy blisters on the body
Mortification - Gangrene of necrotic tissue
Myelitis - Inflammation of the spine
Myocarditis - Inflammation of heart muscles

(N)
Necrosis - Mortification of bones or tissue
Nephrosis - Kidney degeneration
Nepritis - Inflammation of kidneys
Nervous prostration - Extreme exhaustion from inability to control physical and mental activities
Neuralgia - Described as discomfort, such as "Headache" was neuralgia in head
Nostalgia - Homesickness