1) https://en.wikipedia.org/wiki/Mother%27s_curse https://en.wikipedia.org/wiki/Mother%27s_curse 2) By "organism" I assume you mean "cell" since humans have se

1) https://en.wikipedia.org/wiki/Mother%27s_curse https://en.wikipedia.org/wiki/Mother%27s_curse 2) By "organism" I assume you mean "cell" since humans have several thousand different species with their own DNA living on or inside the body at any given moment. We can speak of animal cells, which have two (species and mitochondria) - and plant cells, which have three (species, mitochondria, and chloroplasts). If there can be one two or three, I don't see why there couldn't be even more. 3) Mitochondria are usually sequestered within the cell, which limits their exposure to immune cells. The immune system primarily targets pathogens that are outside the host cells. In fact, some pathogens can exploit mitochondrial pathways to evade immune detection - the most famous of which is HIV.