You are attempting to control an infection outbreak in a city. As the head [of] inoculator(s) in the city, it is your job to manage which antibiotics are purchased, researched, and who is treated. And succeed when everybody's infection is invisible (phase 0).
You are shown the severity of people's infections, in terms of a phase color. The legend at the top of the game interface (and also shown above), tells you the range of the number of bacteria inside that person. The more severe the infection, the greater the chances it spreads to one or more of each person's neighbors as well. Each bacteria is assigned a strength number, by a d10 throw, when it spawns.
You can buy Level 1 antibiotics, and use them on people. If you manage to collect credits (/money), you can first research Level 2 & 3 as well, and then buy & use them. You earn 5 credits each move you proceed.
On pressing done, the move is completed. You get 5 more credits. All the untreated, infections, spread to neighbors with a likelihood, depending on their phase. And the infections also double in number (i.e., the number of bacteria inside them all double).
To inoculate a person, click on that node, and it will turn people, indicating selection. And then click on the antibiotic square, if you've bought one. This will inoculate that person. Inoculating with a level 2 vaccine, involves throwing two d12 for each bacterium in the patient. If the maximum of these rolls, is greater than the bacterium's strength, it will die; otherwise, it will survive unscathed. One d12 die is thrown (for each bacteria inside the host) in a Level 1 antibiotic, and three for Level 3.
Inoculating a person, also prevents his infection from spreading in that move.