Viruses and hacker attacks typically generate a recognizable pattern or \"Signature\" of packets. Most of network traffic analyzer can identify these packets and alert the administrator to their presence on the network via email or page. Most traffics analyzers let them set alarms to be triggered when a particular pattern is seen. Some network traffic analyzers can be programmed to send an email or page when these conditions are met. Of course; this assumes that the virus and its signature have been seen before and incorporated the analyzer's list of packet filters.