The National Science Foundation explains that viruses infect host cells by piercing the cells' outer membranes, digesting the walls and injecting virus DNA into the cells. While many viruses remain dormant under extreme conditions, such as freezing temperatures, they may come to life in the right conditions and enter another organism's cells.
Viruses need a way to enter the body before they can infect a host cell. For instance, WebMD explains that the Ebola virus spreads through contact with the blood or bodily fluids of an infected animal or person. Once this contact occurs, the virus may find a way to penetrate into host cells and spread the infection.
Furthermore, the National Science Foundation finds that viruses, such as certain strains of E. coli, live within parts of the human body, such as the gut, and supply the body with essential vitamins. However, other strains of viruses have negative effects on the body, such as food poisoning. In one study, researchers for the National Science Foundation observed how a structure of the bacteriophage T4, a virus that attacks the E. coli microbe, rearranges itself during a viral invasion to fight the infection. The researchers involved in this study hope to find ways to destroy specific cells carrying these viruses.