Something similar happened in Stephen King's "The Cell." Granted we don't know what exactly happened with We're Alive's Patient Zero, but in "The Cell" the mutation was caused by a cell signal called The Pulse. The people initially corrupted burned out quickly and died/killed each other. Eventually there was an newer more evolved crop from a redesigned Pulse. Maybe the same sort of thing is happening here.
The initial crop of Zombies were attracted by sound primarily and were pretty stupid. They walked past Michael in his car and went to the angry horn guy. Since we first meet them on the freeway, they have evolved into specialties (runners, jumpers, behemoths etc) , and honed their hunting skills.
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