An infection can be defined as the presence of a pathogen in your body. When the pathogen invades your body and replicates, it occasionally leads to certain kinds of damage. This damage occurs either because the pathogen directly attacks the cells in your body or because your body’s immune system creates collateral damage when it fights off the pathogen. An illness, or the sensation of feeling sick, is a result of this damage.
Note that sometimes the illness is not caused by the pathogen but rather by the body’s response to the pathogen. At times the body overreacts, and the extreme inflammatory response to the pathogen is what causes the most harm.
Also note that an infection does not necessarily have to lead to an illness. There are times when the body keeps the infection under control with no damage to the body. An example would be people who are carriers for N. meningitides, the bacteria that can lead to meningitis. Some individuals carry the bacteria around in their noses and respiratory tracts without ever becoming sick.