Final answer:
A pathogen must have a source to cause an infection, and it needs to pass through portals of entry to invade host tissues and subsequently establish an infection. Microorganisms like bacteria and viruses are examples of pathogens, capable of leading to diseases.
Step-by-step explanation:
To cause an infection, a pathogen must have a source. Pathogens can enter the body through several locations that are not covered by skin, which need to have a barrier to prevent infection. These pathogens could be bacteria, viruses, fungi, or protists, and they initiate infections by passing through portals of entry to invade host tissues. An infection represents the successful colonization of a host by a microorganism. Pathogens must achieve exposure, adhesion, invasion, and establish the infection, often leading to disease which is an impairment of the normal state of the living animal or plant body that affects its vital functions. Infectious diseases are caused by these disease-causing agents. Examples of pathogens include primary pathogens like enterohemorrhagic E. coli (EHEC), which is known to produce virulence factors such as Shiga toxin. In contrast, Staphylococcus epidermidis is an opportunistic pathogen that can cause nosocomial infections and is frequently associated with infections transmitted via medical devices.