What is a COD4 infection?

I am not a computer guy. I like to play video games and know nothing about code or CPU lingo. So I was playing COD4 on my xbox 360 for the first time in a couple years and I saw people in online multiplayer falling from the sky and disappearing right in front of my eyes. People were on on top of the map and under the map and on roof tops I have never imagined you could get on. Also there was this annoying colorful writing about game mods that covered a good portion of my upper left screen which made it hard to play. So I ask one of the players that was doing this stuff how he did what he did and he simply messaged me back "Infection Mod." Now I have no idea what that means. I am familiar with the elevator glitch and wall breaches cause I've seen others do those online but I never learned them cause it seemed like it took to much time to practice it so I play straight up. My friend tried to show me an elevator once and it took him forever so I thought it was a waste of time. Is this what this is? an elevator and ground breach? What is an infection mod? What the heck is modding? More importantly, COD4 is one of my all time favorites but it seems to be no fun anymore with this "infection" is black ops doomed as well? IS the COD series ruined? Will this be a problem on MW3? if so I might not buy it.