Virus
Penetration follows attachment: Virions (single virus particles) enter the host cell by receptor-mediated endocytosis or fusion with the lipid bilayer. This is called viral entry. The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose, and fungi one of chitin. This means most viruses can only get inside these cells by force.70 An example would be: a virus travels on an insect vector which feeds on plant sap. The damage done to cell walls would let the virus get in. Bacteria, like plants, have strong cell walls that a virus must get through to infect the cell. However, bacterial cell walls are much thinner than plant cell walls, and some viruses have mechanisms that inject their genome into the bacterial cell, while the viral capsid remains outside.71