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Virus factors encoded in the genome often control the tropism, routes of virus entry, shedding and transmission. In polioviruses, the attenuating point mutations are thought to induce a replication and translation defect to reduce the virus' ability of cross-linking to host cells and replicate within the nervous system. [12]
Many latent and asymptomatic viruses are present in the human body all the time. Viruses infect all life forms; therefore the bacterial, plant, and animal cells and material in the gut also carry viruses. [6] When viruses cause harm by infecting the cells in the body, a symptomatic disease may develop.
The following outline is provided as an overview of and topical guide to concepts related to infectious diseases in humans.. Infection – transmission, entry/invasion after evading/overcoming defense, establishment, and replication of disease-causing microscopic organisms (pathogens) inside a host organism, and the reaction of host tissues to them and to the toxins they produce.
The skin of animals, particularly its surface, which is made from dead cells, prevents many types of viruses from infecting the host. The acidity of the contents of the stomach destroys many viruses that have been swallowed. When a virus overcomes these barriers and enters the host, other innate defences prevent the spread of infection in the body.
In biology, a pathogen (Greek: πάθος, pathos "suffering", "passion" and -γενής, -genēs "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent , or simply a germ .
A human pathogen is a pathogen (microbe or microorganism such as a virus, bacterium, prion, or fungus) that causes disease in humans.. The human physiological defense against common pathogens (such as Pneumocystis) is mainly the responsibility of the immune system with help by some of the body's normal microbiota.
The body's cells constantly display short fragments of their proteins on the cell's surface, and, if a T cell recognises a suspicious viral fragment there, the host cell is destroyed by 'killer T' cells and the virus-specific T-cells proliferate. Cells such as the macrophage are specialists at this antigen presentation. [156]
To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane. A hole forms in the cell membrane, then the virus particle or its genetic contents are released into the host cell, where replication of the viral genome may commence.