why are there flies in an air-conditioned bus?

Just few hours ago, i was in an air-conditioned bus on my way to the library. It started off pleasant until flies began buzzing around me. It was not an isolated case, i'm sure. Cos, i take that bus practically five times a week to school, and these flies were there all the time! Are these flying pests inhabiting every possible hole, every single vent or every little viaduct in the bus? Will it be a case of X-Files where everyone's gonna be trapped within the bus and these little things start swarming out from every possible hole, every single vent or every little viaduct? I shudder at the thought that i might be entrapped in a little "Flies mutiny against Humans" conspiracy here.

Who's then responsible for it?

1)School kids who don't bother to shower before they rush for school, thinking that they smell just the same the nite before because they sleep with their air-conditioners turned on.
2) JC kids who don't bother to shower before they rush for school, thinking that they smell just the same the nite before because they sleep with their air-conditioners turned on. And because they're older, they feel that they have the privilege to consume their breakfast on the bus.
3)School kids who don't bother to shower after their afternoon games, thinking that the air-conditioner in the bus would probably dry their sweat off and render their uniforms wearable the following day.
4)JC kids who don't bother to shower after their afternoon games, thinking that the air-conditioner in the bus would probably dry their sweat off and render their uniforms wearable the following day.
5)Students (and this includes the bunch of JC kids) would can't read simple ENGLISH. So much for getting As for their GPs and As for their O Levels. Have they ever wondered why eating is prohibited on a bus? Maybe they weren't taught that in their JCs or Independent schools. Let's give them a benefit of a doubt and give them a second chance.

JC Boy 1: It's so uncool not to eat on the bus. I'm soooooo busy these days.......Have u finished your tutorials?
JC Boy 2: Man, question 2 is sooooo difficult......Did u play CS yesterday?
JC Boy 1: Yeah....I found a new strategy.... i mean, it's SOOOO uncool to abide all rules and regulations....
JC Boy2: Yeah...Look at my hair....I GOT it dyed off brown....
JC Boy 1: COOOOOL....

If u are already thinking that i have something against JC kiddos, then you are absolutely SPOT on. I absolutely dislike 90% of them. They talk too much (before you start accusing me of being a hypocrite, i'm WRITING here, not TALKING) and they sound like loud-hailers.