Alright... I just want to run this by you guys to make sure what I'm actually thinking is correct. Don't want to go around speaking nonsense and that stuff.

Aeris is half Ancient half Human... Ancients are human as well but had the gift of talking to the planet. The planet comprises of the spirits of the dead.(Lifestream) So she can basically talk to the dead.

Jenova is a sort of virus that travels planet to planet draining life energy wherever it travels. It can change it's form to deceive the inhabitants of those planets as well. So Jenova basically can absorb the spirits of dead people.

Those born or injected with Jenova cells can become host to Jenova.

Not everyone can handle Jenova cells, serious side affects can occur if someone is injected with J-cells. Death can occur as well as being turned into a monster.

Sephiroth was born with the cells of Jenova and therefor can become a host to Jenova.

Mako is another word for the lifestream. A reactor is used to take the lifestream from the earth and use it as an energy source. It's also used to be showered on Soldiers to make them stronger and also used in experiments where mako is heavily used on humans and they're turned into monsters. Mako/Lifestream can also crystalize and turn into materia, or create huge monsters like the weapons.

Please clarify, correct, or expand on anything I've said... Thank you. ^^

I think that's pretty much solid.

Plus, Mako/Lifestream energy is lethal when one was exposed to it too long, as evidenced with Cloud.