With myocarditis, myo- means muscle, card- means heart, and -itis means inflammation. So, myocarditis is inflammation of the myocardium, which is the muscular middle layer of the heart wall, which contracts and relaxes so the heart can pump blood all around the body.
Inflammation in the myocardium layer causes swelling, which damages the heart muscle cells’ ability to contract. That means that less blood gets pumped out of the heart with each heartbeat. If myocarditis is severe enough, it can lead to heart failure, which is when the heart can’t keep up with the demands of the body. Once the inflammation resolves, the heart contraction typically returns to normal, but occasionally, when the inflammation is really severe, it can cause fibrosis, or scar tissue, in the myocardium. Scar tissue doesn’t contract normally, so if that happens, it can cause long term problems with heart contraction.
In North America, viral infections, specifically Coxsackievirus B infections, are the main cause of myocarditis. Viral infections can trigger lymphocytic myocarditis, which is when lymphocytes - the B and T cells of the immune system - and water make their way into the interstitial space - the space in between heart muscle cells. There are plenty of other infectious causes as well, though. One of these is trypanosoma cruzi, a single-cell protozoan that causes Chagas disease throughout South America. In Chagas disease, under a microscope, it’s possible to see groups of amastigotes within the heart muscle cells, which are trypanosomes that are in the intracellular stage. As a result, the heart muscle cells necrose or die. There’s also Trichinella, a roundworm that moves from the intestines into various parts of the body, causing a variety of problems, including myocarditis. Myocarditis can also be seen in Lyme disease which is caused by the bacteria Borrelia burgdorferi, which is spread by deer ticks. Finally, in immunocompromised individuals, there’s Toxoplasma gondii, a single-cell parasite harbored by cats, that can cause myocarditis.