srw
It's a bit more complicated than that...
In the UK, if memory serves, they've traced something like 1100 separate introductions.Don't forget this all probably started with 1 person, the so called 'patient zero'.
It's a bugger of a disease because it can be really nasty if you catch it, it spreads like wildfire if you don't take basic precautions and sits around being infectious for a week before you show any symptoms. But if people take basic precautions - handwashing, staying away from others and wearing masks - its spread is considerably slowed.