Viruses actually are not other cells, they're far, far smaller and simpler then cells, essentially just some RNA or DNA and enough protein to get into the cell and get to work, so it is very worth it for them to infect even tiny single celled organisms, since they are thousands of times larger than viruses, and viruses simply cannot reproduce on their own, unlike bacteria. There are, however, also bacteria that do infect and parasitize other bacteria, growing inside them and using their energy to grow and reproduce, like bdellovibrio bacteriovorus. There are also predatory single celled organisms, that eat other single celled organisms. Single celled organisms come in all kinds of sizes, and fill all sorts of niches in their ecology. Just as a leech cannot take energy and reproduce by photosynthesizing or by eating grass and has to be a parasite, so to do most of these organisms have no choice but to fill the roles they have adapted to fill.