Parasites

Understanding Parasites, Infections, and Immunity

• Parasites are organisms that live on or within another living organism to find a suitable environment and nutrients for growth and multiplication.
• Parasites can be commensal (do not harm the host) or pathogenic (able to cause disease).
• Infection is the process of a parasite entering a host and establishing a relationship.
• The relationship between the host and parasite is determined by the parasite's characteristics and the host's defense mechanisms.
• Entry portals for parasites include the respiratory and gastrointestinal tracts and the skin/mucous membranes.
• Parasites establish themselves by adhering and colonizing body surfaces, which is facilitated by surface components such as pili.
• Characteristics that enable microorganisms to cause disease include pathogenicity (ability to cause infection) and virulence (degree of pathogenicity).
• Virulence is measured in terms of the number of microorganisms or micrograms of toxin necessary to kill given host animals.
• Pathogenicity and virulence depend on invasiveness (ability to enter host tissue, spread, and multiply freely) or toxigenicity (ability to produce toxins).
• Factors affecting invasiveness include surface components that protect bacteria from phagocytosis, the presence of pili, and inhibition of host cell protein synthesis.
• Microbial toxins are of two types: exo- and endotoxins, with different essential features.
• Innate immunity is the first line of defense against infectious agents, is non-specific, and is present from birth.