I don,t think it’s owt to do with how long you have been exposed. For me it’s down to your genetics or health condition.Course I,m no expert but even the experts don,t know.
Those who have been in contact with them should be fine, as WHO have now decided they don't think you can pass it on if you test positive without symptoms. If that's the case it's great news as it's going to make reaching zero infections a day easier and makes a mockery of stories that up to eight thousand people a day get infected with it (if they don't know they have it and can't pass it on the number is redundant).