The invasion and multiplication of microorganisms  or viruses into body tissues. All of the following must be present and in proper sequence to define an infection: (1) the causative agent must be present in sufficient numbers and with such virulence as to destroy normal host cells and tissues. (2) the causative agent must thrive, reproduce, and create a reservoir of more of itself in body tissues, fluids, or waste materials. (3) the causative agent must have a portal of exit, e.g., the respiratory or gastrointestinal tract (4) the causative agent must have a mode of transfer from host to host such as surface contact, aerosolization , fomites (water droplets), or other vector organisms. (5) the causative pathogen must have a portal of entry into the host. (6) the host must be susceptible to the causative agent with portals of entry and inadequate resistance to prevent pathogenic invasion. (Source: