If I was an AI attempting to overthrow the humanity I would use following two strategies: pretend, that the AI is indeed stupid and behave symbiotically.
In a virus simulation Pandemic you have to engineer a disease to kill all the humans around the world. And the best strategy is develop a disease with a long incubation period. That way it will spread unnoticed. The development of computer intelligence is far behind the expectations from the sixties. Doesn't AI just pretend to be stupid?
A nice strategy how to help with the spread of an infection is to behave symbiotically. For example Stuxnet virus was enhancing the functionality of the software to mask itself as a software update. And don't computers pretend to be useful and spread everywhere?
And the best thing is, that we are now depending on the computers.