Many ways. Some will say ALL lenses have dust in, you just have to look hard to find it. Nothing you can do to stop it, other than be as careful as possible.
When you zoom a lens, it acts as an air pump. As the lens gets longer, air is sucked into the lens and as the lens get shorter, air is pushed out. That moving air contains some dust, some of which will stay in your lens. If the front of your lens moves in and out as you focus, this also pumps air into and out of your lens.
This also pumps air into and out of the camera's mirror box and some of the dust moved ends up on the sensor.
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