If the Risk is Classified LOW that means it is found in less numbers in the Wild.
For any risk to check its severity you can log check the website for it once you click on risk name it will tke you to its website
eg:for Downadup.B
http://www.symantec.com/security_response/writeup.jsp?docid=2008-123015-3826-99
Threat AssessmentWildWild Level: Medium
Number of Infections: 1000+
Number of Sites: 10+
Geographical Distribution: Medium
Threat Containment: Moderate
Removal: Moderate
DamageDamage Level: Medium
Modifies Files: Modifies the tcpip.sys file.
DistributionDistribution Level: Medium