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How do bacteria, viruses, fungi, and protozoa cause human infections?
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human disease
The routine monitoring of blood pressure levels is an important part of assessing an individual's health. Blood pressure provides information about the amount of blood in circulation and about heart function and thus is an important indicator of disease.
Infectious agents cause human disease when they overcome host defenses, such as skin or mucosal barriers, and possess sufficient virulence to cause tissue injury. While many microorganisms are commensal or nonpathogenic, they become pathogenic when they invade tissues or are present in sufficient numbers to escape the immune system.1
Mechanisms of Infection
- Bacteria: These organisms induce disease through three primary methods: secreting harmful exotoxins, releasing endotoxins upon the disintegration of the microorganism, or inducing host sensitivity to bacterial antigens.2
- Viruses: Viral disease often results from the virus rapidly harming or destroying host cells (cytopathic effects) by shutting off essential protein or nucleic acid synthesis. Additionally, many viral diseases are caused by the host's secondary immune response, where T lymphocytes or inflammatory processes damage vital tissues while attempting to eliminate the virus.3
- Fungi: Fungal diseases, or mycoses, often occur when host defenses are weakened by factors like malnutrition, cancer, or immunosuppressive drugs.4
- Protozoa: These parasites often utilize complex life cycles and vectors (such as flies or mosquitoes) to enter the host. Some protozoa, like Trypanosoma, evade the immune system through antigenic variation, periodically changing their surface proteins to stay ahead of antibody production.5