"Since Reed's original work, we've found out a lot more about yellow fever. For one thing, we now know that the disease is caused by a virus, something which followed from Reed's eventual demonstration that this was a filterable agent.
Also, we discovered that we could transmit the virus to a variety of animals and animal cells in culture. We've been growing a strain of the virus we call 17D in chick embryo cells for a number of years. We think that this could be the basis of a useful vaccine to prevent yellow fever.
Care to guess how we made the vaccine?