Heart muscle - Myocardium
Inflammation - "itis" (e.g. appendicitis is inflammation of the appendix)
The heart is both a pump, circulating blood to vital organs and muscle and an electrical machine. Interference (illness) can cause pump failure (heart failure with preserved or reduced ejection fraction - hfpef or hfref). Symptoms vary from chest pain (inflammation), shortness of breath/fatigue (decreased pump function), or palpitations/arrhythmia (abnormal heart rhythm).
You may be familiar with the Red Sox starting pitcher, 27 year-old Eduardo Rodriguez, who won 19 games last year and is sitting out 2020 with myocarditis.
Two German studies recently found frequent heart involvement in COVID-19 patients.
In Italy, researchers found an increased number of out-of-hospital cardiac arrests. This didn't discuss any role for hydroxychloroquine, which is associated with heart rhythm disturbances.
The American College of Cardiology has promulgated a general approach to the recovering athlete with COVID-19.
I'd summarize this as "go slow, go low" as far as advancing activity with prolonged convalescence and supervised recovery.
For any individual patient, they should rely on their doctor and specialty consultation as needed.
The point isn't to use 'scare tactics' but recognize that in large populations of athletes during a pandemic, caution is advised. Outcries of "they need to play" or "we want to play" are secondary.
During my career as a Navy physician, we had a gatekeeper role on deciding when athletes could return to play based on medical guidelines, not upon their wishes. This particularly mattered as their primary obligations were to the country as future naval officers.
I am not a cardiologist, but ran an ICU as a Pulmonary/Critical Care specialist in a community hospital for over twenty years.