Simulating the spread of a virus in a computer network

In this paper we propose a simulation problem that can be used as an exercise in a simulation class using an Excel add-on simulation package. Suppose that N objects (computers, people, animals, plants) are interacting in pairs. Every time unit (for example, a second for a computer, a day or a week for people), each object randomly selects another object and contacts it. Suppose that one object is infected with a virus. How long will it take until all objects are infected? How many objects are infected after 5 time units? This process is a simplified model for the spread of AIDS among humans, computer viruses among computers, or any communicable disease among humans, animals, or plants.