i don't remember some things

im now at episode 16 of gurren lagann but i watch the show very infrequently so i don't remember much of the plot, i remember who is versus who but not why(i remember that at episode 2 we learn about the beastmen and that they attack the humans on the surface periodically but not why), i will not ask for explanation of everything that happened so far(especially since most of it probably doesn't matter anymore since lordgenome has been killed) but there are some things that i still want to know: why were the humans trying to reach the teppelin? and who is viral anyway? and there was another beastman that i think commanded viral at episode 13, who is he? is there more information about them other than that they worked for lordgenome?