In some cases, the specific adhesion provides a molecular explanation for host range. Specific colonization factors provide a potential target for intervention either as vaccines or as treatment using receptor analogs. If colonization is prevented, disease is usually prevented. In the absence of an immune system, the host is just a happy meal. One key nutrient is iron which the host chelates.
Some bacteria possess systems to compete with the host for iron.
Depending on the source, the term invasion may have two meanings. However, another meaning has arisen which is to enter host cells. I'll try to use the term, bacterial-mediated internalization, rather than invasion. Pathogens have evolved with their host, and transmission is critical for the survival of the pathogen. If disease does not result in transmission to another host or in the increase in the number of the pathogen, then it is likely that the infection is not relevant in an evolutionary sense.