well blacknmild that really depends on which source of zombie informtion your going from
on the Romero Series the dead are either actual walking corpses for which the cause is neither explined, or those tht were bitten or attacked by zombie which then become zombies. No one kn ows the source, but the back of the cover reads a radioactive sattelite, but the remake sys nothing of the sort
in the return of the living dead series there is a chemical called Trioxin which reanimtes corpses and can turn the living into zombies withtoo much exposure,bites from Trioxin Zombies also cause Zombie-ism
in Resident Evil there are a number of biological agents tht cause Zombies, the T-Virus, the G-Virus, those leaches, Los Plagas which is a Psuedo-Zombieness
in Night of the Comet it was cused by the dust left by the Comet
In Night of the Creeps, Slither, and the Faculty it is caused by some Biological agent from outerspace
in the Zombie Survival Guide bd World War Z(im assuming i havent red world war z yet) its caused by a Virus called Solenum
in 28 days later it is caused by a virus called Rage
in the Walking Dead comic and graphic novel series(great read) the cause is unknown, however Regrdless how you die you will become a Zombie, that is unless brain function has been terminated. There is a chapter where character is bitten but his leg is removed to see if it willprevent the turn, however this is inconclusive because he died of bloodloss.
In the Re-animator seires Herbert Walker's reanimtion serum recreates Zombies
In Evil dead 1, 2 and Army of Darkness they are caused by the side of Evil
In Shaun of the Dead they pretty much follow Romero rules, once again they aren't explined,various jokes and references to different Zombie media
including Rage infected monkeys
In Eternl Darkness they are the minions of Cosmic dieties
there are other movies where Zombies are caused by any sort of magic, necromancy, Voodoo
thats all i can think of for now
also, there is a difference between undead and zombies, but this line can be hazy and is crossed several times
in most media, your screwed if your bitten, but like any infection it might be curable through thurough sanitation or amputation, in 28 weeks later your defintely screwed unless you have genetic immunity.