Infections with viruses can cause a wide variety of diseases. All viruses are very small, too small to be seen without a microscope, and have a simple structure consisting of a nucleic acid genome encapsulated in a protein shell (capsid) and enclosed in a lipid membrane. The nucleic acid may be DNA or RNA.
Viruses cannot reproduce on their own, but they can enter cells where they multiply and take over the host’s cell machinery. Unlike other living organisms, they do not produce adenosine triphosphate, a chemical that drives many cell functions, and they do not have ribosomes to make proteins. They also lack the cell’s cell wall and mitochondria, which provide energy for cellular processes. Viruses are therefore classified in a gray area of the taxonomy of living organisms and are generally considered not to be true organisms, but rather genetic parasites.
Viruses are so tiny that they can only be seen with electron microscopy. This is one of the main reasons it took scientists so long to recognize and confirm their existence, as they could not be grown in a culture medium like bacteria. They were also too small to be retained by filters made from diatomaceous earth or kieselguhr, which were widely used at the beginning of the 20th century for distinguishing between bacteria, fungi and protozoa. Another defining characteristic of viruses is that they are not retained by filters made from diatomaceous or kieselguhr clay, which were commonly used at the time to filter water.