According to Microsoft, your virus-infected computer is no different from a diseased human. This means that it should be quarantined until it's healthy so that it doesn't put others at risk:
Just as when an individual who is not vaccinated puts others' health at risk, computers that are not protected or have been compromised with a bot put others at risk and pose a greater threat to society. In the physical world, international, national, and local health organizations identify, track and control the spread of disease which can include, where necessary, quarantining people to avoid the infection of others. Simply put, we need to improve and maintain the health of consumer devices connected to the Internet in order to avoid greater societal risk.
Quarantining infected computers doesn't exactly sound like an easy feat, but Microsoft has a plan. It wants to implement a "health check" that confirms whether a computer is ready to connect to the Internet:
Although the conditions to be checked may change over time, current experience suggests that such health checks should ensure that software patches are applied, a firewall is installed and configured correctly, an antivirus program with current signatures is running, and the machine is not currently infected with known malware.
There are problems with this plan, though. Some people might not want to jump through the hoops required for a health check or their computers might be too infected to even run the necessary application. Either way, Microsoft appears to think that we can turn the internet into a safer place if we all just get tested before connecting. [Technet via BBC]