INTERACTIONS BETWEEN DISEASES

HOW CAN SYSTEMS THINKING HELP TAKE INTO ACCOUNT THE INTERACTIONS BETWEEN DISEASES?

The classic connection between diseases was Edward Jenner’s observation that children who develop cow pox were very unlikely to get smallpox even when exposed. This fundamental observation led to the concept and term “vaccination,” from the Latin word “vacca,” or cow. It also established that there is a relationship between diseases.

In recent years, it has been increasingly recognized that some diseases predispose to other diseases. In addition, there are patterns of risk factors or symptoms that tend to occur together. These are often called syndromes. As we have seen, the components of the metabolic syndrome frequently occur together and greatly increase the probability of coronary artery and other large blood vessel diseases. The recognition of the frequent occurrence of the metabolic syndrome has led to concerted efforts to identify individuals with the syndrome and make a multi-intervention approach to reducing the risk.

HIV provides a good example of the complex interactions that occur between diseases. A number of sexually transmitted diseases, especially those that interrupt the mucosal membranes lining the genital organs, such as syphilis and herpes genitalis, increase the risk of being infected with HIV if exposed. In addition, diseases such as gonorrhea greatly increase the level of the HIV virus that appears in semen, thus increasing the communicability of HIV.

HIV itself predisposes to a large number of infections, the most important of which from the public health perspective is tuberculosis. Finally, HIV is found in association with other conditions, including drug abuse and intimate partner violence, which greatly increases the burden of disease. These types of interactions of HIV with other diseases and conditions have been described as a syndemic. A syndemic is the occurrence together of two or more diseases that interact to magnify the occurrence and/or burden of disease.13

Disease interactions are not always detrimental. At times, one disease may provide protection against other diseases. Early infection with bacteria and other pathogens in environments such as that which occurs on farms has been shown to be associated with reduced incidence of food and skin allergies.

Systems thinking can not only help us understand the relationship between diseases, but it can also help us understand the impact that a disease has over the life span.