"If you recover from yellow fever, you will be immune to the virus for the rest of your life." - http://www.bupa.co.uk/individuals/he...y/yellow-fever

It's not like Malaria (which is not a virus) where you have it possibly forever. You have yellow fever, you get sick, you get over it. Like Chicken Pox, measles, and certain other viruses that have vaccines.

So if you visited a Yellow Fever country 2 years ago you have no way to transmit it to South Africa.