A virus is an infectious agent that needs a host to live.
Although viruses aren't said to be living, they do have genetic material, whether that be RNA or DNA. They cannot reproduce on their own, but they can produce more viruses by injecting a cell with their genetic material. Although when sketched or animated they may seem like living organisms, they're technically not living organisms.
A bateriophage is one example of a virus, and it infects bacteria. It has structures called "tail fibers," but they aren't legs. They also have a head-like structure, but it's not a head. The point of this last paragraph is to explain how their appearence can be decieving.
To get more information about viruses, look more into the bacteriophage or HIV. They're both caused by viruses.