are thinking, i have to keep an eye on that one. >> there is always the unknown. you have to keep your options open. we have learned to expect the unexpected. there are new diseases with new twists in terms of exposure, et cetera. i am more worried about the things we don't know about. knowing about these viruses is irstthe f step. before some of these pathogens reach the lab, they come here. david mckenzie has been taking us on this journey with a team of south african virus hunters. the last stop is here, a level 4 bio lab in south africa. the only lap of its kind on the after can continent.