What is a common characteristic of ssRNA viruses?

Single-stranded RNA (ssRNA) viruses are characterized by their ability to exist in two different forms known as positive-sense and negative-sense RNA. Positive-sense RNA can serve directly as messenger RNA (mRNA) and can be immediately translated by the host cell's ribosomes into proteins. In contrast, negative-sense RNA must first be transcribed into a positive-sense RNA by an RNA polymerase before it can be translated into proteins. This dual characteristic of existing as either type is crucial for understanding the viral life cycle and the mechanisms of viral infection and replication, making it a fundamental aspect of ssRNA viruses.

The other options do not accurately describe ssRNA viruses: they do not always have a double-stranded configuration, as their defining characteristic is being single-stranded. While some ssRNA viruses may have a helical structure, not all of them do, as they can also form icosahedral shapes. Additionally, ssRNA viruses are indeed able to replicate within host cells, as they depend on the host machinery for their replication and protein synthesis.