Complex sentence: An echovirus measures 24-30 nanometres (nm), and is similar to other viruses, since it has a naked protein capsid, which makes up 75% of the virus particle that encloses a dense central core of single-stranded RNA.
Split into simpler sentences: An echovirus measures 24-30 nanometres (nm), and has a naked protein capsid, which makes up 75% of the virus particle. The capsid encloses a dense central core of single-stranded RNA.