Q. Is a camera memory card a possible source of infection? I sent one from my digital camera to a supermarket photographic service to have some pictures printed. After its return I found a Trojan on my computer.

A. Congratulations on spotting the Trojan! It is certainly possible for a memory card, or any other storage device, to be infected. Proving that your card was infected by the supermarket would be very difficult because there are so many possible avenues - e-mails, websites, downloads, swapped memory sticks - through which malware might infect a computer. The lesson, of course, is to always use anti-virus software and to scan anything that you connect to your computer, or download onto it.