Viruses are part of nature and are found in all cells. Their number by far exceeds the number of all cells and organisms on our planet. Some viruses, such as flu viruses or SARS-CoV-2 (coronaviruses) cause diseases, others are harmless. How is this possible? Can harmful viruses be eradicated? To answer these questions, we need to understand viruses better, so we first examine what actually are viruses? And how do they multiply and change? With our research, we try to predict how cells react to viruses. In this way, we search for new drugs against viruses.