The alien want to make some new virus (or editing the gene of existing virus) to increase human death rate, and they don't want human to know the virus has been edited. I have read some similar conspiracy theory, such as the SARS conspiracy. Some scientists said SARS is man-made while some said it is not man-made. How can scientists know whether the virus is man-made? What kind of changes in genetic structure is impossible to happen in the natural environment?
If you want to know more about the background:
Aliens from another universe came to the earth. Some functions in the human brain are useful to them. (The details are not related to the question.) People whose brain is to be used by the aliens will be dead. The aliens don't want people to know of their existence, so they don't want to use living people directly. Should the alien do so, then there would be many strange deaths occurring and human may discover the truth. So the aliens want to use "clinically dead" people. There is a 10 minute window of for the aliens to make use of the brain before brain damage occur. (The alien don't need to go to the scene and take out the brain from the human body.)
Quote from the second link:
Once someone has stopped breathing they have about 4-6 minutes until some brain damage begins to occur. At around 6-10 minutes some brain damage is likely. And, generally speaking, after 10 minutes, irreversible brain damage is almost certain. However, as mentioned earlier, there's no way to know with certainty so don't assume and stop rescue efforts.
To sum up, the aliens want more people to die, however, they want "sustainable development", the alien only want to increase the death rate instead of eliminating all humans.