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Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.
When RNA viruses invade the human organism, they inject their RNA into the cytoplasm of the host cell. Once they are inside the cytoplasm, RNA can be used to synthesize proteins, and, eventually, to form replica viruses. There is a special category of RNA viruses, which is called retroviruses. These viral agents contain protein known in biology as reverse transcriptase. Once the retrovirus injects its RNA into the cell, reverse transcriptase transcribes the RNA into DNA (the operation that is the reverse of normal transcription).