Infections caused by viruses, bacteria, parasites, or fungi in different parts of the body, as well as some autoimmune diseases, can lead to myocarditis. Viral infections are the most common cause of myocarditis. However, bacteria causing diphtheria and Lyme disease, and systemic diseases like lupus, can also cause myocarditis. In addition, certain medications, antibiotics, and drug use that cause toxic reactions can also lead to inflammation of the heart muscle. If a person has a strong immune system, the system fights off microbes without issue. However, individuals with weakened immune systems may fail to do so. Heart muscle inflammation can also result from fungi.