The medicines used in homeopathic form consist of two classes. One class includes those substances obtained from the natural world of plants, minerals, and animal products. The second class, called nosodes, includes substances derived from disease products, tissue samples, mucus, pus from discharges, or pure cultures of microorganisms. Nosodes correspond to the specific diseases associated with the individual bacteria or virus, or the infectious material sample taken from a patient. Both of these classes have been used to prevent disease. Examples of this include Lathyrus sativa (a plant) for polio and Pertussin (a preparation of the bacteria Bordetella pertussis) for whooping cough.

A medicine that has proven effective for a specific epidemic of a disease in the community can be used as the preventive for other cases of that disease, though homeopaths tend to use those medicines that have proven themselves in the past. As a general rule, homeopaths utilize the nosode of the infectious organism to prevent disease. Nosodes are named with the Latin terms for the infection or organism, Morbillinum for measles, Diphtherinum for diphtheria.

This method of homeopathic prophylaxis has been formulated into strategies and rules of two types - short-term prevention during epidemics and long-term prevention.

Experience with the use of nosodes during epidemics has led to a level of confidence and optimism about the protective effect of this method. Since the mid-nineteenth century, homeopaths have attempted to prevent or limit the spread of disease during epidemics, with some success. Most of the experience with this approach occurred during the era preceding the availability of vaccines. Homeopaths reported a decrease in the severity and frequency of disease in those patients who received the nosode preventively.

The method of homeopathic prophylaxis has never been rigorously tested. Nonetheless, there is some evidence suggesting that homeopathic medicines do act to prevent diseases during epidemics. One study observed the occurrence of meningitis in a group of children who received a homeopathic preventive (Meningococcinum 10c in a single dose) during a 1974 epidemic in Brazil. Of the 18,640 children given the homeopathic nosode, 4 developed meningitis (0.02 percent), compared to 32 cases in the 6,340 unvaccinated children (0.5 percent). This represents a significant difference in a controlled study, although the control group was not randomized (Castro & Nogueira, 1975). Eisfelder reported an uncontrolled study of 50,000 children who received Lathyrus, a homeopathic preparation used to treat paralysis, in varying potencies during the polio epidemic of the 1950s. Only one of these children developed (non-paralytic) polio. The general population had a significantly higher rate of polio than 1 in 50,000 (Eisfelder, 1961).

These studies do not prove the effectiveness of homeopathic prophylaxis in epidemics, but many homeopathic practitioners have been convinced by their own experience with this form of disease prevention. The practice of using homeopathic preparations to prevent disease during epidemic exposure may be effective. The medicines cause no adverse effects, and, in the absence of any other form of prevention, there was no reason not to use them. In an epidemic of a serious disease their use is still warranted, though there are valid reasons to allow children to undergo the milder childhood occurrence of measles, mumps and chickenpox to acquire lifelong immunity.