biosüsteem, mille pärilikkusaine moodustab kas DNA või RNA ning mis paljuneb ainult elavates rakkudes, ära kasutades rakkude toitumissüsteeme, üliväike rakulise ehituseta objekt, haigusetekitaja, geneetilise tasandi parasiit, kes paljuneb ainult elusrakkudes, a minute infectious (agent) characterized by a lack of independent metabolism and by the ability to replicate only within living host cells, any of a large group of submicroscopic infectious agents that are usually regarded as nonliving extremely complex molecules, that typically contain a protein coat surrounding an RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth and multiplication only in living cells, and that cause various important diseases in humans, animals, and plants