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.
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.