Viruses are small strands of hereditary material surrounded by a protein coat that can only reproduce inside living cells. They come in various shapes and sizes and are too small to see without a microscope. Viruses demonstrate the single characteristic of life which is reproduction. They enter cells, inject their DNA, use the cell to replicate themselves then burst out to infect more cells. Viruses can lie dormant for years before activating. Vaccines work by introducing weakened virus particles to trigger antibody production to fight future infection. While there are no medicines to directly kill viruses, the body produces interferons and gene therapy works to replace infected DNA and allow cells to function normally again.