Pneumonia is just a lung infection, so I imagine there's a number of ways you can make a person to unknowingly inhale something.
However, it looks way too complicated for a plot. There are many tried and proven methods of getting rid of people. Spooks aren't actually that good in hiding their works - we know about a lot of cases where people were assassinated (of course, we may also not know about many, but I think we have a good sample). Among those, we have a lot of ways it can be done - shootings, stabbings, explosions, poisons, drownings, falling from heights, whatever - but I can't remember any case where a biological agent were used. And thinking about it - biological agents are hard to produce, hard to handle, unstable, unpredictable in use, can't be properly targeted, why would anyone use that instead of dozens of easier and more common methods?
So while it does look suspicious on its face, I'd have hard time believing it's an example of an assassination.
However, it looks way too complicated for a plot. There are many tried and proven methods of getting rid of people. Spooks aren't actually that good in hiding their works - we know about a lot of cases where people were assassinated (of course, we may also not know about many, but I think we have a good sample). Among those, we have a lot of ways it can be done - shootings, stabbings, explosions, poisons, drownings, falling from heights, whatever - but I can't remember any case where a biological agent were used. And thinking about it - biological agents are hard to produce, hard to handle, unstable, unpredictable in use, can't be properly targeted, why would anyone use that instead of dozens of easier and more common methods?
So while it does look suspicious on its face, I'd have hard time believing it's an example of an assassination.