peritonitis, pleurisy, and pericarditis are all the inflammation of the outer lining of pelvis and abdomen, lungs, and heart respectively.
In the case of peritonitis the lining of the abdomen and pelvis is inflamed due to infection (bacteria or parasites) or injury. In the case of pleurisy, what happens is that the outer lining of the lungs is filled with fluid making it difficult and painful to breathe (so that's a pretty bad pain). There are many causes for this condition. In the case of pericarditis, the lining of the heart is inflammed and some fluid might be accumulated, therefore the heart has trouble functioning (also very bad pain)
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