In my world there is a rogue scientist who wants to wipe out at least 90% of humanity. He's has a sample of measles which are very contagious but he needs something new. Could he merge measles with Ebola or Rabies to create disease which is both airborne and very lethal? It's only one virus that can be possibly recombined with the others to achive the end to my story. Please for explanation of chimera- is it possible to do it ?
Mixing measles and Ebola kind of smells like coating a nuclear bomb with nerve gas. Too much!
Better for your narrative is to mix it with something unexpected. For a story, I like the idea that a harmless pathogen should confer the ability to do great harm.
Here is how it would work.
Rhinovirus has an arsenal of poorly understood tricks to evade the immune system, over and over. They are poorly understood because rhinovirus infections are generally so harmless.
When correctly merged with measles, the rhinovirus can slip the measles past the immune system in someone who has been vaccinated.
The deadly part is when the measles comes out. The immune system is faced with a sudden, overwhelming measles infection. The consequent overwhelming immune reaction leads to measles encephalitis. Thus the fatality rate is near 100% in people with immunity to measles, either via vaccination or prior infection. Nonimmune persons get a regular case of measles.
There are stray survivors, all of whom have low immunity for one reason or another: recent liver transplant, advanced HIV, extreme old age. But as a population, only the unvaccinated survive this scenario. Three-quarters of the way thru this story, a pissed off crew of Orthodox teenagers from Brooklyn show up at the villains lab and sort out his stuff.
Your logic is wrong
In reality, the more deadly a virus is, the less deadly it is because people will actively protect against it.
If a sudden new disease starts killing people, government groups like the CDC will enact quarantine and start working on a cure. People will avoid going out and wear masks and other protective clothing etc.
Someone dies coughing up blood and the whole hospital will be shut down, every person who came into contact will be isolated and tested. If it escapes, people will isolate themselves in bunkers if needs be until the disease dies out or is cured.
To wipe out most of the human population, the disease needs to be basically harmless thus doesn't warrant notice until it's too late.
To do this you start with a cold virus and have it make people sterile. People will be over the disease and may not notice they can't have kids anymore until years later.
This way the disease has years to spread and scientists may not even find out what caused the sterility in the first place.