Can someone please tell me what causes acute myocarditis?!

Myocarditis refers to the inflammation of the heart muscle.

Inflammation is one of the body's responses to antigens.
When a bacteria, virus etc invades the heart muscle, the body begins to stimulate (via bradykinin, prostaglandins, histamine etc) pain receptors which cause vasodilation of the blood vessels at the scene. This widening of the local blood vessels attracts large numbers of phagocytes.

Normally, inflammation would trigger either the complete reconstitution of the damaged tissue, scarring of the tissue (bridging gaps between tissues or angiogenesis), abscess formation or chronic inflammation.

Chronic inflammation is one of the ways the body can fight itself (auto-immune disorders). Although, myocarditis may be short term and be the effects of the body fighting off an antigen such as a virus or bacteria (which is why some treatments are aimed purely at the virus causing the inflammation, for example).

Vasodilation brings phagocytes (such as neutrophils) to the region - in this case, the heart muscle.
Inflammatory mediators (especially released from neutrophils) can cause damage to the heart muscle.
For instance, the neutrophils may release toxic oxygen metabolites (e.g hydrogen peroxide or free radicals) which can cause tissue destruction and blistering or they may release elastase which destroys connective tissue and further blisters the skin.

Over time, the heart's muscle will begin to degrade which may cause a myocardial infarction.

Most cases are viral (Coxsackie B virus)
Additional causes:
-Other viruses like Flu, Measles, hepatitis, or chicken pox
-Bacterial infections, such as tetanus, gonorrhea, or tuberculosis
-Parasite infections, such as Chagas' disease (which is caused by an insect-borne protozoan most -commonly seen in Central and South America)
-Rheumatic fever
-Surgery on the heart
-Radiation therapy for cancer that is localized in the chest, such as breast or lung cancer
-Certain medications
-Illegal drugs and toxic substances

http://medical-dictionary.thefreediction…

The answers in answer 1 makes toxins like substances and other changes in Heart Muscle cells so that the body does not recognize it as of its own. So it causes an allergic like reaction so the Heart Muscle cells become inflamed.