Flexi Says: Both viruses and bacteria can cause infectious diseases in humans and other organisms. However, they do not share many other similarities. Structurally, viruses and bacteria are very different. Bacteria are living cells. Viruses contain DNA but lack the other parts shared by all cells. Therefore, viruses are not cells. Viruses cannot reproduce by themselves. Instead, they infect living hosts, and use the hosts’ cells to make copies of their own DNA.