Really dumb (probably) questions about plagues and research

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Ok. I haven't really started my first real game of SEIV. In order to learn about the game a little bit better I set it up so as to avoid alien contact as much as possible. I wanted to learn more about what you can do outwith alien interaction, and then I will start learning how the diplomacy and intelligence and stuff works. Might sound a bit dull to some, but hey, whatever floats your boat...

Game rules: Infinite resources; dumbest AI; no AI assistance; largest size Spiral Arm map; no warp points; 100,000 tech points start; medium tech cost; low tech knowledge at start

Up until my Last couple of turns, whenever I encountered alien controlled systems I immediately closed the warp points. I was the definitive xenophobe.

I have around 30 or so systems under my control, and they are all just about converted to my (terran) natural habitats of optimally conditioned oxygenised planets and gas giants.

I have cleaned out my tech tree for research, and am building intel facilities like a mad KGB leader could only dream of. Replacing all of my research facilities with intel ones, in preparation for when my race aren't a bunch of xenophobic gits (they'll just become really sneaky scheming gits instead)...

aaaaaaanyways. i cam across a minor race (i ditched the xenophobic policy when i built a few dreadnoughts that were basically floating death barges - no kidding these things make borg cubes look like freighter escorts ), and offered them a protectorate treaty which they accepted. not long after that one of my planets suffered from a level one plague. how the hell do i cure it? the population of the planet are getting annoyed and around 12m people are dying each turn. i have looked at every single facility i can build and there is nothing there about preventing or curing plagues or diseases, and the manual offers bugger all help...

and in regards to research. is replacing all my research facilities prudent even though i have finished the tech tree? will new branches of the tree open up if i steal technology from another race, or will i just steal one off technologies? will there be any techs to steal, or i have already discovered everything that i could possibly steal anyways?