When you breathe in viruses after someone sneezes uncovered, the viruses enter your cells, reproduce, release trillions of copies of itself, and proceed to take over other cell. Sometimes viral DNA simply embeds itself in your own human DNA, where it can lie dormant or sometimes come back to life when you least want it, as occurs with recurring cold sores, shingles from a long-past chicken pox, and even some cancers, especially if you immune system is weakened. This is what can happen with Kaposi's sarcoma in immunodeficient patients infected with HIV. Sometimes, a viral code can end up in the DNA in your sperm or eggs, which then gets passed on to future generations. Viruses are champions of DNA mutation, able to carry, exchange, and modify the DNA between cells or from one species to another (e.g., the spread of antibacterial-resistance genes from one bacterial organism to its own species and then on to other bacteria types). More tomorrow.