Final answer:
Viruses are often contracted through friendly means, such as direct contact, indirect contact, or vector transmission. Cell-surface receptors on host cells also play a role in virus binding and infection.
Step-by-step explanation:
Close to 75% of all viruses are contracted through friendly means.
Viruses often bind to cell-surface receptors on the host cell. Chemical differences in the cell-surface receptors among hosts mean that a virus that infects a specific species (for example, humans) often cannot infect another species (for example, chickens).
Direct contact, indirect contact with fomites, or transmission through a vector, such as mosquitoes or ticks, are common ways in which viruses are transmitted.