Viruses are constantly swarming into our bodies. Sometimes they make us sick; sometimes our immune systems vanquish them; and sometimes they become a part of ourselves. A type of virus called a retrovirus makes copies of itself by inserting its genes into the DNA of a cell. The cell then uses those instructions to make the parts for new viruses. HIV makes a living this way, as do a number of viruses that can trigger cancer. On rare occasion, a retrovirus may infect an egg. Now something odd may happen. If the egg becomes fertilized and gives rise to a whole adult individual, all the cells in its body will carry that virus. And if that individual has offspring, the virus gets carried down to the next generation. At first, these so-called endogenous retroviruses lead a double life. They can still break free of their host and infect new ones. Koalas are suffering from one such epidemic. But over thousands of years, the viruses become imprisoned. Their DNA mutates, robbing them of the abil