Microscopic, extremely pathogenic particles (lat. virus = mucus, sap, poison), which are among the most dangerous enemies of the vine. Simple viruses usually consist of a protein envelope and nucleic acid enclosed in it (in contrast to the proteinless viroids). All viruses contain the program (some viruses also contain other auxiliary components) to propagate and spread them. They do not consist of a cell and therefore do not have an independent replication nor a metabolism of their own and are therefore dependent on the metabolism of a host cell such as a bacterium.
According to common doctrine, viruses are not considered to be living beings, but they can at least be considered "close to life". So far 3,000 different species have been identified, but many more probably exist. They are only viable on the living tissue of a host and convert its metabolism. The contents of the host cell are completely dissolved and degenerate into a malignant cell.