What do virus threat levels mean?

The Microsoft Product Support and Services Security Team assigns a severity rating for viruses, worms, and other technological attacks according to their impact on Microsoft software and customers. The Microsoft Security Response Center uses this three-level severity rating system for Microsoft security bulletins. A virus, worm, or other threat receives a severity level rating based on whether it has one or more of the listed conditions below.

Critical

• We have discovered a vulnerability related to Microsoft software that could allow a virus or other malicious program to harm your system or personal files or to steal personal information stored on your computer.
• An update to fix or protect against the problem may not be available yet.
• There are two or more known ways for the threat to infect people's computers or a new (as yet unknown) path of infection is possible.
• If your computer becomes infected, it could quickly spread that infection to other computers (via e-mail or instant messaging or through a local network) without your knowing it.
• Important system and personal files on your computer could be irreparably damaged.
• Because of this threat, you might not be able to connect to the Internet at all, or important online services or Web sites might become unavailable.

Moderate

• We've discovered a potential vulnerability related to Microsoft software, which means a risk to users could develop but has not occurred yet.
• There are two or fewer known ways for the threat to infect people's computers, or a new (as yet unknown) path of infection is possible.
• The risk of infection spreading quickly is medium to high.
• We do not have indications that unique data destruction has happened or will happen because of this threat.
• No significant disruption of local network or Internet service has occurred.

Low

• No vulnerabilities related to Microsoft software have been found.
• There is only one known way for the infection to spread, and the potential for the infection to find new ways to spread is low.
• Therefore, the risk of this threat spreading far and wide is relatively low.
• There's no sign that unique data destruction has happened or could happen.
• No significant disruption of local network or Internet service has occurred.