About a week ago I came across a Marvel magazine that was a special edition specifically for Avengers: Infinity War. It provides the history of each Infinity Stone in the MCU up to Infinity War, has interviews the actors, etc..
when it talks about the Reality Stone, it says the following:
The Reality Stone is unique, even by the standards of the Infinity Stones, in that it is not really a stone, but is in fact, a red, viscous liquid. There are, however, special theories that state that at one time, long ago, it was actually a stone, just like the others.
The [Reality Stone] seeks out a host body and will draw strength from its life force, eventually killing the host. Lesser beings will sicken and die if exposed to it for long. But Malekith was a strong host, so it enhanced his strength, giving him the power to fight Thor on an equal basis.
If this is true, then, how is the Reality Stone actually a stone now, and able to be seated in Thanos's gauntlet like all the others?
Per the events of Thor: Dark World, and of the provided passage from Marvel themselves, the Reality Stone isn't really a stone, and, it has the behavior of "infecting" a host, just as it did to Jane Foster and Malekith.
So, why didn't it do this to Thanos?
Clip of Malekith extracting the Aether from Jane Foster's body, and accepting it into his:
Thanos using the Reality Stone and it seated in the Gauntlet, just like the others stones: