Introduction to infectious diseases: microorganisms, normal microbial flora, routes of transmission, virulence factors, pathogenesis, signs and symptoms, diagnosis, treatment and prevention. For patient education.
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Infectious diseases are disorders caused by microorganisms, or microbes, such as viruses, bacteria, fungi, or parasites. There are trillions of microbe species, but only several thousands are diseasecausing organisms, or pathogens. In fact, many species live inside or on human body, constituting the socalled normal microbial flora, which is not only harmless but also helpful in protecting the human host from invasion by pathogens.
Infectious diseases can be acquired in different ways: through direct or nondirect contacts with an infected person, via consumption of contaminated foods, having a contaminated medical device, from an infected mother to the child during pregnancy or birth, via contacts with an infected animal or animal waste, or through bites from carriers, such as mosquitoes or ticks.
The pathogens can be inhaled in contaminated aerosol droplets, consumed in food or drinks, or enter the nose, mouth, or eyes as people handle contaminated objects then touch their face. Organisms that spread through body fluids are usually transmitted via sexual contacts, blood transfer or skin wounds.
The normal flora is usually harmless, but can cause disease in people with a weakened immune system. Resident bacteria may also cause infection if they enter parts of the body that are supposed to be free of germs, which can happen during an injury or surgery. Overuse of antibiotics may kill protective bacteria, allowing opportunistic pathogens to thrive and cause disease.
The ability of an organism to infect and damage a host is determined by its virulence. The higher the virulence of a pathogen, the more severe the disease it can cause. Virulence factors are molecules or structures that enable the pathogen to invade host tissues or evade immune defense.
Some bacteria use adhesive hairlike projections, called fimbriae or pili, to bind to host cells and establish a starting point for tissue invasion; others use specialized molecules, called adhesins, for the same purpose. Viruses use spike proteins on their surface to bind and enter host cells.
Some bacteria have a capsule that helps them resist being “eaten” by phagocytes. Others produce enzymes or toxins that prevent phagosome maturation or counteract the lytic effect of lysozyme.
Pathogens are capable of interfering with virtually any aspects of the host immune system. Among other mechanisms, they can impair antibody production, or produce superantigens, which induce nonspecific activation of Tcells and massive cytokine release.
Some viruses can remain dormant inside host cells, effectively hiding from the host immune system.
Bacteria may develop resistance to antibiotics, thus evading treatment.
Uncontrolled growth of pathogens destroys host tissues, producing symptoms. Infection triggers the body’s inflammatory response, which brings immune cells to the site to fight the invader. While inflammation is an important defense mechanism, it may become excessive and cause damage to the body’s own tissues, contributing to the severity of the disease.
Many bacteria release exotoxins that cause damage to body systems. Most toxins are produced during the course of infection, but there are also preformed toxins, such as those responsible for food poisoning, that can evoke disease even in the absence of the bacteria.
Gramnegative bacteria have an endotoxin as part of their outer membrane. This endotoxin is responsible for excessive release of inflammatory mediators and procoagulant factors that can be lethal in septic patients.
Diagnosis is based on symptoms and lab tests that indicate the presence, and possibly, identity of the pathogen.
Antibiotics, antivirals, antifungals, and antiparasitics are used to treat infections by bacteria, viruses, fungi, and parasites, respectively.
Transmission of infectious diseases can be largely minimized by practicing personal hygiene, especially hand washing. Vaccines, when available, are the most effective prevention measures. Antibiotics may sometimes be given for prophylactic purposes.