From Wikipedia:
Epidemics of the 19th century were faced without the medical advances that made 20th-century epidemics much rarer and less lethal. Micro-organisms (viruses and bacteria) had been discovered in the 18th century, but it was not until the late 19th century that the experiments of Lazzaro Spallanzani and Louis Pasteur disproved spontaneous generation conclusively, allowing germ theory and Robert Koch’s discovery of micro-organisms as the cause of disease transmission. Thus throughout the majority of the 19th century, there was only the most basic, common-sense understanding of the causes, amelioration, and treatment of epidemic disease.
The 19th century did, however, mark a transformation period in medicine. This included the first uses of chloroform and nitrous dioxides as anesthesia, important discoveries in regards of pathology and the perfection of the autopsy, and advances in our understanding of the human body. Medical institutions were also transitioning to new hospital styles to try to prevent the spread of disease and stop over crowding with the mixing of the poor and the sick which had been a common practice. With the increasing rise in urban population, disease and epidemic crisis became much more prevalent and was seen as a consequence of urban living. Problems arose as both governments and the medical professionals at the time tried to get a handle on the spread of disease. They had yet to figure out what actually causes disease. So as those in authority scrambled to make leaps and bounds in science and track down what may be the cause of these epidemics, entire communities would be lost to the grips of terrible ailments.
Common diseases that went by other names in the 19th century.
| 19th century | current |
| green sickness | anemia |
| cramp colic | appendicitis |
| black death, typhus | bubonic plague |
| Asiatic cholera | cholera |
| dropsy, anascara | congestive heart failure |
| costiveness | constipation |
| croup, hives, choak | croup |
| putrid fever, membranous croup | diptheria |
| bloody flux | dysentery |
| puerperal toxaemia | eclampsia |
| dropsy of the brain | encephalitis |
| podagra | gout |
| erotomania, nymphomania (f), satyriasis (m) | hypersexuality |
| gripe | influenza |
| mania | insanity |
| bilious fever | liver disease |
| ague, congestive fever, chill fever | malaria |
| rubeola | measles |
| brain fever | meningitis |
| dropsy | oedema, ascites (morbid accumulation of water in the tissues & cavities) as a symptom of kidney disease, heart failure, liver disease, chronic lung disease |
| lung fever | pneumonia |
| puerperal mania, insanity, melancholia | postpartum psychosis |
| childbed fever | puerperal fever, postpartum inflammation |
| scarlet fever, malignant quinsy, putrid sore throat, canker rash | scarlatina, cynanche maligna |
| variola | smallpox |
| apoplexy, convulsions | stroke, cerebral hemorrhage |
| siriasis | sunstroke, heat stroke |
| the great pox, French disease | syphilis |
| quinsy | tonsillitis, suppurative |
| scrofula | tuberculosis of the lymphatic glands |
| enteric fever | typhoid, typhoid fever |
| camp fever, ship’s fever, spotted fever | typhus |
| lues venera | venereal disease |
| chin cough | whooping cough |
| Yellow Jack, Yellow Plague, Bronze John, dock fever | yellow fever |
Sources:
Common Diseases in the 19th Century
Nineteenth Century Disease (very detailed, with references)
A 19th-Century Mother’s Handwritten Record of Her Babies’ Childhood Illnesses