Strength varies among the walking dead. While those that have been re-animated for a time tend to get weaker those freshly turned tend to be relatively strong as their muscle is still intact and they have not yet begun to decompose.
While no one can be 100% sure most scientist track the origin of the virus back to the illegal "Beserker" nerve gas used by Iran on it's on citizens during a civil war in 2052. No one knew who created the gas and no one was rushing to claim it either.
The gas had the effect of driving it's victims insane and causing them to attack one another until all were dead. The rage was so strong that some bodies were known to continue moving for up to an hour even after they were dead.
It is believed that those animated bodies came into contact with the living and infected them. Most likely through a bite. At that point after entering the blood stream the dead's saliva containing elements of the Beserker gas mutated into what has become called " The Beserker Virus". This virus brought the bitten victims back to life with an insatiable need to feed on the living.
While again no one can be sure of this it is the most believed and substantiated theory to date.