Heart disease, of course, has long been considered an ailment of exclusively human causes, rising in the developed world as traditional infectious disease has waned. Heart disease kills more people than any other condition worldwide, including 47 percent of Americans and Europeans and 30 percent of all global deaths. Scientific consensus points to some combination of behavioral and environmental links (smoking, diet, exercise, stress, and so on down the list), along with various genetic components. Except in rare cases of acute infections, such as infectious endocarditis, microbes have been considered irrelevant.

But in 2013, an infectious component revealed itself, in the form of trimethylamine-N-oxide, or TMAO. TMAO isn’t a bacterium itself; rather it’s created when bacteria digest carnitine, a compound found in meat, and lecithin, a fatty substance common in certain foods such as eggs, milk, and some nuts. In research published in Nature Medicine and The New England Journal of Medicine, a team led by Cleveland Clinic’s Stanley Hazen found that human subjects with the highest levels of TMAO in their blood had about twice the risk of having a heart attack, stroke, or death compared to those who had the lowest TMAO levels.

The chain of causation here requires a few clever links. First, the hypothesis goes, the human eats a diet high in meat or lecithin. The gut bacteria feed on carnitin and lectithin and release a substance that in the human liver is turned into TMAO. This excess TMAO allows cholesterol to get into artery walls and also prevents the body from shedding extra cholesterol. Once there, the cholesterol accumulates on the blood vessels, causing atherosclerosis. Hazen’s research is only suggestive; it needs further replication in more human studies. But it suggests a profound departure from our conventional understanding of heart disease, and what role bacteria may play.

Other areas of noninfectious disease have also turned out to have infectious aspects in recent years. The best known example is that of cervical cancer, which was first associated with the human papillomavirus, or HPV, in the 1970s. A vaccine against the virus was developed in 2006, and is now frequently administered to young women before they become sexually active. These and other preventive efforts have helped reduce the rate of cervical cancer by more than 70 percent since the 1970s. But it turns out that HPV might be a more active virus than we have accounted for: It is likely also associated with rising rates of throat cancer, anal cancer, penile cancer, and other cancers. Though HPV is associated with women, a surprising 12,000 men each year develop an HPV-associated cancer, according to the CDC.

A similar blurring of the lines has happened in research into gastrointestinal and autoimmune diseases, driven by the growing awareness of a microbiome—those one trillion bacteria that live in our guts and on our skins, a trillion organisms that unwittingly affect the larger organism that is the human body. A study released last November suggested a strong connection between a bacteria named Prevotella copri and rheumatoid arthritis. Not only did researchers establish a significantly higher rate of the bacteria in humans with the disease, but when they administered doses of P. copri to mice the animals developed increased inflammation.