The game models (VERY loosely) 4 months in a virus-infested city; 4 weeks to a month, and 7 days - the smallest unit of time - to a week.
At the start of the game, the city is populated with a number of people, some of whom are infected. The player assumes the role of a health care team that can cure infected people.
The player selects a difficulty level at the start of the game; difficulty is a function of the initial number of persons, probability of infection at the start of the game, and probability of infection on contact. Each level of difficulty has a different set of these factors.
Every (game time) day, each person moves in a random direction to an adjacent block that is unoccupied. Should the person attempt to move in a direction that would take the person beyond the city boundaries, moves in a different direction.
If a naïve (previously uninfected) person collides with an infected person, they become infected; any cured persons are assumed to have acquired immunity from previous exposure and are unaffected.
The health care team can move freely and can occupy the same square as a person. If there is an infected person in that square, that person is cured.
The health care team is represented by a white icon with a red cross. Persons are represented by icons whose color / avatar is a function of clinical status: naïve (never infected) - blue, infected (currently) - red, or cured - green.
The objective is to "cure" infected persons, and to minimize the number of infected persons at the end of the game (preferably zero).