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The term "pathogen" is most often used for agents that disrupt the normal physiology of a multicellular animal or plant. However, pathogens can infect unicellular organisms from all of the biological kingdoms (see Viruses, below).
The human body has many natural defenses against some of the more common pathogens (such as Pneumocystis,) in the form of the human immune system and by some "helpful" bacteria present in the human body's normal flora. However, if the immune system or "good" bacteria is damaged in any way (such as by chemotherapy or HIV, or by antibiotics being taken to kill other pathogens,) pathogenic bacteria that was being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infections.
Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics and fungicideFungicides are pesticides for destruction or development prevention of fungi. One example are the QoI. See also: List of fungicides. Pesticide., pathogens continue to threaten life on EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla, and can be particularly devastating to its humanHuman beings are defined variously in biological, spiritual, and cultural terms, or in combinations thereof. Biologically, they are classified as Homo sapiens ( Latin for knowing man , a primate species of mammal with a highly developed brain. In spiritua inhabitants.
To this day, only one virus has been declared entirely eradicated from the planet - Variola virus, which causes SmallpoxSmallpox (also known by the Latin names Variola or Variola vera is a highly contagious disease unique to humans caused by two virus variants called Variola major and Variola minor. major is the more deadly form, with a typical mortality of 20-40 percent o, through the use of vaccination. A massive undertaking to eradicate polioPoliomyelitis ("polio") is a viral paralytic disease. The causative agent, a virus called poliovirus (PV), enters the body orally, infecting the intestinal lining. It may proceed to the blood stream and into the central nervous system causing paralysis an through vaccination is in progress, but in some developing countries large amounts of people still have no access to the vaccine. In recent years, some Western families have opted against vaccinating their children for either religious reasons or because of mistrust in the effectiveness or safety of vaccination. (This dropoff in vaccination has been said to have contributed to the sharp rise in recent measlesMeasles also known as rubeola is a common disease caused by a virus of the genus Morbillivirus''. Reports of measles go back to at least 700, however, the first scientific description of the disease and its distinction from smallpox is attributed to the M cases in the United Kingdom.)