" email@example.com" wrote in news: firstname.lastname@example.org:
Yes they will spread on a LAN where the machines are sharing resouces once the malware has reached a machine and is executed.
There are self populating worms that will seek out and infect other machines that it finds unprotected and drop its payload once it is executed. That's the vast majority of unsolicited inbound traffic that will hit a firewall and is blocked are other machines that have self populating malware seeking other machines on the Internet that are not protected and infect them.
It doesn't matter if the machine is on a Local Area Network (LAN) or Wide Area Network (WAN/Internet) as they are both networks and if the machine is open to malware attacks, it doesn't matter if it's a dial-up, ADSL, Broad Band cable or whatever.
Usually, malware attacks the machine because the user has contributed to it in someway with the happy fingers that click on unknown links, go to dubious WEB sites or click on and open unknown email attachments. Once, malware has reached the machine and is executed, it's over.