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When an intracellular parasite goes to enter a host cell, it is particular about the type of host cell. This is because most intracellular parasites are able to infect only a few different cell types. [21] Viruses use a number of host receptors to gain entry to the cell, usually by causing endocytosis. [7]
In their Parasite Immunology article on worms and viral infections, Kamal et al. explain why some parasitic worms aggravate the immune response. [13] Because parasitic worms often induce Th2 cells and lead to suppressed Th1 cells, problems arise when Th1 cells are needed. [13] Such cases occur with viral diseases. [13]
A parasite may evolve to become less harmful for its host or a host may evolve to cope with the unavoidable presence of a parasite—to the point that the parasite's absence causes the host harm. For example, although animals parasitised by worms are often clearly harmed, such infections may also reduce the prevalence and effects of autoimmune ...
Major groups of parasites include protozoans (organisms having only one cell) and parasitic worms (helminths). Of these, protozoans, including cryptosporidium, microsporidia, and isospora, are most common in HIV-infected persons. Each of these parasites can infect the digestive tract, and sometimes two or more can cause infection at the same time.
Viruses, however, use a completely different mechanism to cause disease. Upon entry into the host, they can do one of two things. Many times, viral pathogens enter the lytic cycle; this is when the virus inserts its DNA or RNA into the host cell, replicates, and eventually causes the cell to lyse, releasing more viruses into the environment.
Bacterial virulence factors include adherence factors to attach to host cells, invasion factors supporting entry into host cells, capsules to prevent opsonization and phagocytosis, toxins, and siderophores to acquire iron. [15] A photomicrograph of a stool that has shigella dysentery. These bacteria typically cause foodborne illness.
Parasites are organisms which derive sustenance from its host while causing it harm. [2] The study of parasites and parasitic diseases is known as parasitology. [3] Medical parasitology is concerned with three major groups of parasites: parasitic protozoa, helminths, and parasitic arthropods. [2]
There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise. [25]
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