It's hard to say which kind of liquid is the first one.. I think it's a virus (but it also kinda looks like water though!)
1. A = either water or a virus
B = Blood
C = Bacteria
D = Water/Virus and Bacteria
2.The mouse dies after being injected with substance B and D and lives after injected with substance A and C
3. I think that if you inject the mouse with a different blood type it will not survive, just like with humans the blood will clot which can cause veins to close.
And I think with the last one; if you have a virus or caught a bacterial infection you would create anti-bodies/white blood cells to defend the body and be resistant to them. But when you combine them the virus will take the bacteria as a "host" to make more virus cells and if you don't take antibiotica (like in the mouses case) eventually he would get really sick.
Hope these are the awnsers you are looking for : )