Relationships of the parasites with their respective hosts are called parasitism. There are two important processes that play a significant role in parasitism, infection and intoxication.
Infection refers to entry of a pathogen into host tissues after transmission, and its growth and reproduction resulting in disease. The events of infection are as follows:

Attachment and colonization of the pathogen:

The host contains its own normal microbial flora. A pathogen enters into host and compete normal microbial flora. Pathogens re equipped with certain specialized structures (haemagglutinin, fimbriae, capsule, lectin, mucus gel, pili, receptors etc.) that provides high degree of specificity to a particular tissue and help in colonization. These specialized molecules or structures are also known as adherence factors or adhensins.The adhensins bind to complementary receptors site present on cell surface of the host.

Entry of the pathogen:

After attaching to the surface of epithelium, pathogen enters deeper in to the epithelium through a. producing lytic substances and dissolving host tissues, b. disrupting the cell surface, or c. degrading carbohydrate-protein complex between the cells.
After entering inside host tissues the pathogen disseminate throughout the whole body. This is accomplished by specific products or enzymes that help in spreading.

Growth and multiplication of the Pathogen:

After different mechanisms ultimately the pathogen reaches to terminal lymphatic capillaries surrounding the epithelial cells, which merge in to large lymphatic vessels. The lymphatic vessels connect the circulatory system and discharge the pathogen their in. Finally, pathogen reaches to all the body organs. The pathogen gets proper nutrients, optimum pH and temperature in certain specific areas. This results in growth and multiplication of the pathogens. Some pathogens have evolved specificity to multiply either inside host cells, in tissues, in blood plasma, and also can generate its own nutrient gathering mechanism. Sometimes the metabolic products of blood become toxic and result in a condition called septicemia.

Intoxications

Intoxications are the diseases caused due to entry of toxins (Latin toxicum means poison) into the host systems or body organs. Intoxications may occur in the presence or absence of the pathogen. The conditions resulted from a toxin is called toxemia.Toxins are of two main types: exotoxins and endotoxins.

Exotoxins: Exotoxins are heat labile, soluble proteins secreted by the pathogen that after release is circulated to other part of body tissues or target cells. After treatment with formaldehyde the toxicity of exotoxins is lost but their antigenic properties are retained. They have ability to induce the production of antibodies i.e. antitoxins which react with toxins and neutralize them. In this inactivated form these exotoxins are known as toxoids. Toxoids are used as vaccines to immunize against several diseases e.g. tetanus, diphtheria, botulism etc. The general characters of exotoxin are : a. synthesized by exotoxin genes present on plasmid of the pathogen, b. heat- labile protein which becomes inactivated between 60°C- 80°C, c. toxic in low doses, d. specific in their action, e. easily inactivated by iodine, formaldehyde etc. to form toxoids, f. highly immunogenic and induce the production of neutralizing antibodies, g. named after the name of disease botulinum toxin, diphtheria toxin etc. and h. of different types based on their mode of action such as neurotoxins (affecting nerve tissue), cytotoxins (affecting general tissues), enterotoxins (affecting intestinal mucosa).

Endotoxins: The outer membrane of cell wall of most of the Gram-negative bacteria consists of liopopolysaccharides (LPS). LPS is toxic in certain conditions and therefore it is called endotoxin because it is tightly bound to cell wall and released only after lysis of microorganisms. Lipid A portion is the toxic component of LPs. Endotoxins are- a. heat- labile, b. toxic only at high doses, c. capable of producing systemic effects such as fever, shocks, blood coagulation, weakness, diarrhea, inflammation, and fibrinolysis.

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