About condoms and STDs?

i was reading the wrapper on a condom i have in my drawer and it says something like that condoms are 80% effect against stds and pregnancy, im just curious about how we know that. How do they get the number 80%? is that just a disclaimer? how come condoms cant fully protect against these things? i mean i know they can pop but what if they dont. i know this sounds gross but after everytime i have i go fill the condom with water and see if it leaks. It never has so far for me, and if the condom doesnt leak and no semen gets out on my shaft and into the , then is it still possible to have gotten that girl pregnant, and if so how? Also about STDs, speaking hypothetically, if a girl has an std and you have with her, if your condom never pops, and none of your bare skin makes contact with her or any of her fluids (not likely but just speaking hypothetically) would you be totally safe from catching the STD? I know this may sound like a weird/stupid question but regardless a lady came to our school to talk to us about std's and condoms and i asked her this question and she could not answer me. Also why is there no cure for aids yet? i know theres alot of theories about it, even some conspiracy theories, but can we even prove that aids comes from hiv 100% of the time? How come hiv is only spread through bodily fluids, its a virus, and all virsues that use a human host are contagious, so how come the rest of us havent gotten it by now? i know these may sound like stupid questions to some people but like i said we werent educated very well on the subjects of std's and pregnancy at school. please inform me!