Children increasingly use computing devices. However, it is unclear whether they have basic knowledge of security-related issues such as computer viruses and, in case they do not, what they can learn about them. In this study, we first examined preconceptions of computer viruses among Czech and German primary school students using a newly developed test. Second, the experimental group (but not the control group) received an intervention to learn about computer viruses, antiviruses, and software updates by means of a 45-min lesson combining a cartoon series on viruses, frontal instruction, and discussion. Both groups again completed the test. A preliminary analysis indicated that Czech and German children already knew key points concerning computer viruses. However, overall, their knowledge was patchy, and children also had various misconceptions. The experimental group improved from pre- to posttest (d = 1.06), while this was not the case for the control group. These initial results indicated that knowledge of computer viruses should and can be taught to primary school children.