Delivering a single virus to a cell allows the virus to infect the cell, replicate, and give rise to many progeny viruses. These viruses can then infect many neighboring cells.
In this animation, you can see how one S. typhimurium invades an epithelial cell of the intestinal tract, survives the intracellular defense mechanisms of the host cell, and multiplies.
When two different strains of influenza infect a single cell, their genetic material can mix freely, resulting in a new third strain of influenza.
Bacteria can transfer genetic material, and thus drug resistance, to other bacteria via conjugation.
Watch this animation to see the molecular tricks that an infectious strain of Escherichia coli uses to infect your gut.