Stalker,
Undefined,
17 years ago
For the first stage of its metamorphosis the alien would more than likely require nutrients supplied by the body of the living host in order to properly develop & grow, although once it had matured into the "adult" chestburster stage, even killing the host would be insufficient, & as Alien Resurrection proved, hosts carrying a chestburster are almost impervious to harm.
It is quite likely that the chestburster secretes some kind of adrenaline-like substance into the host's bloodstream which enables them to survive punishment a normal member of its species would not be able to sustain, this acts as a way of helping to protect the burster until it has matured. During the final stages of the chestburster's life cycle, moments before it punches its way through the ribcage of its unfortunate victim, the host can become rabid & aggressive, even attacking & killing anything around it to preserve its own survival if it is threatened by any external danger (Dr. Wren was the cultivator of this behavior in Alien Resurrection, & was savagely beaten by Purvis, despite shooting him several times).
Severe head trauma however, such as a bullet to the brain, will still kill the host in this state, as was proved in AVP when Sebastian was shot dead by Lex moments before his body was chestbursted, also proving that if a burster actually reaches maturity, even killing the host will not destroy it.