Thunderbird Contamination

[Jim]

FC 8 , Thunderbird 2.0.0.14

I have a friend that has about 300-400 old email messages laying in his 
inbox of Thunderbird and when he tries to read
New emails they are mixed in with old ones and some of them are blank, 
it just a big box of sh-t .
I removed his emails and AddressBook from Thunderbird, and deleted and 
reinstalled his AddressBook and old emails
back into the new Thunderbird , everything was working for a few hours 
and then here we go again.
Are there Viruses that affect  strictly  Email  Browsers  like  this ??
This always seems like that he goes through this everytime he gets a 
large amount of Emails stored  in his Inbox.