Explain Something I Missed (Spoilers)

The movie was ok but something just doesn't align; I think I must have missed something here

Air Force sends a ship to another planet
Crew becomes infected and dies
Ship crashes into Italian sea
Boy finds cylinder with the alien monster inside it
While in hospital the very sexist air force pilot tries yelling at his partner who is dieing


Why did certain people become infected? Was this supposed to have been a side effect from the alien or from the alien planet itself? Was it ever explained why some become infected and others were not (how disease traveled)

Why in the world were the humans so antagonistic to the alien? He really appeared scared / lost and was hardly aggressive to anyone

For that matter what caused that crazy elephant to rampage and just keep attacking the alien

Why can Air Force pilot not tell the nurse (almost a doctor) any info, yet they had no problems whatsoever talking with a Sicilian fishing village full of people about what they were looking for and offer a reward?

Minor technical detail; if creature grew so quick what was he eating? He went from 6 inches to at least 30 feet in a matter of hours