Fossils could not be found in that layer of rock
A virus<span> is a small </span>infectious agent<span> that </span>replicates<span> only inside the living </span>cells<span> of other </span>organisms<span>. Viruses can infect all types of </span>life forms<span>, from </span>animals<span> and </span>plants<span> to </span>microorganisms<span>, including </span>bacteria<span> and </span><span>archaea
</span>While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles<span>, also known as </span>virions<span>, consist of two or three parts: (i) the </span>genetic material<span> made from either </span>DNA<span> or </span>RNA<span>, long </span>molecules<span> that carry genetic information; (ii) a </span>protein<span> coat, called the </span>capsid<span>, which surrounds and protects the genetic material; and in some cases (iii) an </span>envelope<span> of </span>lipids<span> that surrounds the protein coat when they are outside a cell. The shapes of these virus particles range from simple </span>helical<span> and </span>icosahedral<span> forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an </span>optical microscope<span>. The average virion is about one one-hundredth the size of the average </span>bacterium<span>.</span>
