Yahoo mail is not plagued with the virus. In fact, Yahoo and Hotmail are probably the safest, because they use up-to-date scanners on all mail going through their servers.
MyDoom doesn't hijack email addresses, it - like most other email virusses - fakes the sender address. After infecting a computer, it takes names from the infected computer's address book, creates an email with one of the addresses faked as the sender, and sends to other users in the address book. It's done to throw people off the scent, so the virus can run longer without being detected.
Example: Person A's computer gets infected. Person A has B and C in his address book. The virus sends an email to C, with B faked as the sender. C, a typical user, tells B his computer is infected. Meanwhile, A goes on merrily sending infected emails.
The only conclusion one can draw here, is that the infected computers apparently have a lot of Yahoo contacts in their address books.
