I only played the free version, review it.
You are given a choice of disease carriers. For me, the simplest was a bacterium (it is positioned as such, but there are methods of use that are universal for all options), the most difficult was a nano-virus (since, unlike other options, they begin to interfere with you immediately, and without waiting for symptoms to appear ).
During the game there is a good tutorial that is very hard to miss. The world is divided into regions, generally corresponding to the countries of the world or their associations in one geographical group (after all, the pixels are not infinitely small :)). To win, you need to kill all people, which requires developing your disease. To do this, you need to develop skills in spreading the disease, since some territories can be reached by a limited number of methods (modifiers also allow you to complicate the game - for example, in the description of a modifier that blocks the spread through water, the question is asked: “And how will you infect Greenland now?”. Answer I won’t give it). Lethal symptoms must then be developed, which can be combined in curious combinations, but to do this the mild and non-lethal symptoms must first be developed. However, a disease has three parameters - likelihood of transmission, lethality and visibility. The higher the probability of transmission, the faster people will contract the disease (within the available methods of transmission). The more deadly the disease, the faster people will die, but the more noticeable it is, the sooner they will start looking for a cure. It’s suspicious when the whole country started having headaches and vomiting on the same day :)
For me, the optimal strategy turned out to be to quietly infect everyone, and then quickly develop fatal symptoms. But it took me some time to get to this point, plus, depending on the type of host, you need to develop different methods of transition or start killing earlier.
Later media, for which you need to pay, add new disease settings, but see for yourself :)