There’s no question that people can be infectious before they show symptoms. The question is how long before you show the symptoms, and there’s a huge debate about that.
The World Health Organization thinks it’s about 48 hours before you show symptoms. Some people say it’s 12 hours or 24 hours. It’s uncertain, and this really goes to the issue of who you come in contact with during that time, say a day or two days. In Australia, people are focusing more on the day before. I suspect you should really focus on two days before but that increases the number of contacts that you have to get a hold of.
Dr Norman Swan, Physician and Journalist