Because viruses are microscopic particles consisting of nucleic acid molecules – (DNA or RNA), enclosed in a protein shell that can only live inside living cells of the body. By nature, viruses are autonomous genetic elements that have an extracellular stage in the development cycle. Viruses are obligate parasites, as they are not able to reproduce outside the cell (it does not have its own reproduction mechanism). After penetrating into the cell, the virus embeds its DNA in the DNA of the cell into which it penetrated, into the cell genome.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.