Symantec speculates that the threat -- initially named Trojan.Horse but renamed to W32.Imsolk.A@mm -- originated from a botnet and appears to be hitting "many, many companies indiscriminantly."
"Once the threat copies itself to another machine, if a user even opens the folder that contains the threat on this new machine, this will launch the threat and cause it to spread further through both email and over shared drives," the company wrote in a bulletin.
Department of Homeland Security officials are looking into the virus and the U.S. Computer Emergency Readiness Team and DHS National Cyber Security Division are expected to issue a bulletin later today. They will also conduct forensic analysis to try and determine where the worm originated. A DHS official said that several federal departments and agencies are experiencing the virus, although the official would only confirm NASA.
"US-CERT has received multiple reports from a number of federal agencies and private sector entities experiencing an email worm...US-CERT is in the process of collecting and analyzing samples of the malware and has developed and disseminated mitigation strategies," said DHS press secretary Amy Kudwa in a statement.
A spokesman for the Florida Department of Transportation e-mail has been taken down at the agency because of the spam attack. He said six other agencies in Florida have also been hit by the virus.
While the trojan hindered communications, it hasn't had a major impact on operations, he said.
"It's not life and death, a bridge hasn't collapsed and killed anybody," said Nelson Hill, chief information officer for the Florida Department of Transportation.
Adobe systems on Tuesday advised computer security experts that there were vulnerabilities in the Adobe reader software, noting that hackers were looking to actively exploit a recently detected vulnerability. This could explain why the e-mail was being sent in a .pdf format.
As IT professionals raced to address the problem, annoyed employees took to Twitter to rant.
"Office servers offline, due to spam assault. No e-mail for anyone. Now maybe I can get some work done!" posted one employee.
"The world is coming to an end. The "here you have" email virus just took down times square," joked another.
NASA's Lunar Science Institute tweeted, "Houston, we have a problem... it's called spam."
ABC News' Jason Ryan, Marisa Bramwell, Lee Ferran and Sidney Wright contributed to this report.