Yeah, the flu spread like any other illness exacerbated by the contained, close quarters, environment and likely unsanitary living conditions and hygiene practices. Remember, everyone already has the "zombie virus" and you don't die from it on a bite. You die from pathogens and bacteria carried by a rotting corpse biting you. (though, presumably, a recently turned walker will turn you so there may be another element in play.)
As to the gun cache in Alexandria, while they may have a plentiful supply at this point it's likely they won't for very long. It's entirely possible The Saviors will "vet" Akexandria, find the weapons cache and relieve them of it. (It's hard to believe The Saviors would "allow" someone under their control to have such a supply and they already know Alexandria stole a lot of their weapons.)
So unless Rick and the Alexandrians manage to hide the weapons somewhere The Saviors won't find them they likely won't have them very long.
As to the gun cache in Alexandria, while they may have a plentiful supply at this point it's likely they won't for very long. It's entirely possible The Saviors will "vet" Akexandria, find the weapons cache and relieve them of it. (It's hard to believe The Saviors would "allow" someone under their control to have such a supply and they already know Alexandria stole a lot of their weapons.)
So unless Rick and the Alexandrians manage to hide the weapons somewhere The Saviors won't find them they likely won't have them very long.
The fuel they have is better suited to be used in normal cars, at best A Team them up by welding steel plating to the car and go all Mad Max on your enemy, should protect well enough against normal ammunition ((it worked for the A Team
).