virus are the invaders that need a host in order to survive. they are either made of dna or rna, when they get inside the human body, they attack the genetic information of the cell and force it to make many copies of virus. they may destroy the whole cell when they are about to leave or else they may add some of it glysoprotein that can form spikes outside their surface. viruses are different because others may be not be able to transform tissues of the body meaning that they cauaes mutation.