Here's an interesting research report from Vanderbilt University as reported in Science Daily on July 12, 2016: https://www.sciencedaily.com/releases/2016/07/160712130109.htm A few quotes: "Summary: A research team has identified some of the methods that allow mutant mitochondrial DNA to act selfishly by circumventing the molecular mechanisms that cells use to regulate mitochondrial activity. Mitochondria are special organelles found in cells that produce most of the chemical energy that powers cell operations. Mitochondrial dysfunction has been associated with a wide variety of illnesses, including autism, Alzheimer's disease, schizophrenia, dementia, Parkinson's disease, epilepsy, stroke, cancer, chronic fatigue syndrome and cardiovascular disease. Now, a team of researchers at Vanderbilt University has discovered that mutant mtDNA may cause diseases by behaving "selfishly" -- in a fashion that benefits them while harming their host. Mitochondrial disorders have some unusual properties. "Unlike bacterial infections that tend to be all or nothing, mitochondrial infections can range from zero to 100 percent," said Patel. "This makes mitochondrial disorders multi-symptomatic, with a lot of individual differences. One person with a mutant load of 50 percent might be symptom free while another person with 80 percent might have severe symptoms."