Got to multiply and make enough of the microbe to bother the host. The microbe must find a site that fulfils its metabolic needs. The sites certain microbes find hospitable varies. Most human pathogens are mesophiles, neutrophiles, microaerophils or anaerobes, and chemoorganotrophs. Think about the conditions in the human body.
Colonization is the replication of microbes to form colonies in a hospitable environment.
Remember that all mutualistic and commensulistic bacteria do this adherence and colonization routine, too. The last, but required, part of infectious disease process is invasion. To be a pathogen and to cause infectious disease, a microbe must destroy host tissue, called invasion. Invasion differentiates normal flora, the microbes that live harmlessly adhered and colonized in or on your body, from pathogens that cause disease. Pathogens have virulence factors, substances made by pathogens that damage host tissue, or promote adherence and colonization that disrupts normal host cell function.