Your company network has been infected with a virus. You have cleaned all PCs, workstations, and servers. However your network still keeps getting re-infected with the same virus. Which of the…

UPS (Uninterruptible Power Supply) devices are purely electrical hardware designed to regulate and supply backup power; they contain no writable storage medium or operating system capable of storing or executing viral code. Because a virus requires a persistent, writable environment to embed itself, a UPS device is fundamentally incapable of acting as a re-infection vector.

Downloading infected email attachments is a well-documented and common re-infection vector, meaning this CAN be a cause and is therefore not a valid answer to 'cannot be a cause'.

Modern network printers contain embedded operating systems, persistent flash storage, and sometimes hard drives, all of which can store and spread malware, making them a legitimate re-infection source.

Network Interface Cards operate at the physical and data-link layers using firmware stored in read-only memory that typical malware cannot modify or infect. Because NICs have no user-accessible writable storage and no execution environment for arbitrary code, a conventional virus cannot persist on or propagate from a NIC.

Infected USB drives and other portable media are a classic and highly common re-infection pathway, reintroducing the virus to the network each time the media is connected to a cleaned machine.