According to Nature, viruses do not have ribosomes. Therefore, they cannot form proteins from messenger RNA; this is why viruses must have host cells in order to replicate. Biology 4 Kids indicates that viruses also lack nuclei and mitochondria. Cytoplasm is missing from some viruses, as well.
Though viruses are responsible for a wide range of illnesses --from the common cold to HIV-- they have other similarities. Nature.com explains that they are all extremely small. In addition, Biology 4 Kids points out that viruses also lack the ability to respond to their environment. Until they are wiped out, they simply carry out their essential functions.