Both viruses arrive in the form of an infected Word document attached to email. When the infected document is opened, the virus infects Word's global template, Normal.dot. Once the global template is infected, all future Word documents will be infected.

To identify infected emails, look at the subject line and body text of the message:
The subject line of Melissa.u is "pictures" and the body is "what's up?".

The subject of Melissa.v is "My Pictures" and the body is blank.

Both variants delete data and spread very rapidly.

Melissa.U invokes a MAPI email client and sends itself to the first four email addresses in your Address Book (including distribution lists). It then attempts to render your system inoperable by deleting the following system files:
c:\io.sys
d:\command.com
d:\io.sys
c:\Ntdetect.com
c:\Suhdlog.dat
d:\Suhdlog.dat

Melissa.v invokes a MAPI client and sends itself to the first forty addresses in your Address Book. It then attempts to delete files and directories in the root of mapped drives with the following letters: M, N, O, P, Q, S, F, I, X, Z, H, and L.