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 disease-causing organisms, or pathogens. In fact, many species live inside
or on human body, constituting the so-called 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 non-direct 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 hair-like 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. Some organisms produce
enzymes to cut across host cells to make entry from mucosal surfaces. 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 super-antigens,
which induce non-specific activation of T-cells 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 pre-formed toxins, such as those
responsible for food poisoning, that can evoke disease even in the absence of the bacteria.
Gram-negative bacteria have an endotoxin as part of their outer membrane.
This endotoxin is responsible for excessive release of inflammatory mediators
and pro-coagulant factors that can be lethal in septic patients.
Different diseases manifest differently but the most common sign of infection is fever,
which is actually one of the ways the body uses to fight infection. Other symptoms vary depending on
the system that is affected. A systemic body-wide infection may lead to sepsis and septic shock,
with rapid heart rates, breathing rates, decreased blood pressures, and signs of organ damage.
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. In some cases, the body immune system successfully
destroys the pathogen and infection resolves on its own. In other cases,
the pathogen overwhelms the immune system, and medical treatments are required.
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.