What is R and D?
Research and development. When you’re looking to develop a new drug or a vaccine, you need a couple things. One is you need to take the small sample of the pathogen (e.g. virus) you’ve been provided and make many copies of it. The way you do that is to give it cells it can infect so it can make copies the way it normally does. Once you have many copies of it (we’re talking billions of copies), you can take some and experiment on them to see how it reacts to different things.
The other thing you need to do is see how different compounds/drugs/etc. alter its behavior. For instance, if you think zinc might block viral replication inside human cells, you can take some stem cells, add some zinc to the environment, then add the virus and see how its replication is actually impacted. This avoids the issue of a living human being becoming infected, sick, and possibly dying from the experimentation and allows you lots of freedom to experiment with drugs or compounds that might otherwise be dangerous inside a living human being.
The reason fetal stem cells are so often used is that they replicate extremely quickly, reliably, and for a very, very long time. If you took cells from an adult human being, they’re specialized cells (which can behave differently), they replicate far fewer times (essentially they die out quickly), and it takes a lot longer to build up a large supply of them. There are other alternatives involving certain types of animal cells or other cell types that don’t come from an aborted fetus. Each presents its own set of challenges, but typically you CAN use one or more ethically derived cell lines to do your research, understanding there are some caveats to each. Or you can choose to use fetal stem cells and do everything faster and cheaper, but also with the moral and ethical issues attached to those.
I hope that’s helpful.