Now I'm imagining self-replicating hair tangles, akin to how a prion causes proteins to misfold. Take care not to get to close to someone infected with one or the knot might brush your hair and induce it to knot in the same manner.
The nightmare scenario would be a screaming mass of people bound together helplessly by their intertwined hair knots, binding anyone who comes close to attempt a rescue into the same hellish ball. The only solution would be to hose them down with industrial-strength Nair from a safe distance.