The term infection refers to the state where a host organism has been invaded by another organism, typically a microorganism such as a virus, bacterium, protozoa, algae, or fungus. The invader is able to elude the responses of the host that are designed to kill it. Strategies include rapid multiplication, which can overwhelm the host defenses, or escaping from the host's immune system by multiplying inside host cells.
The second aspect of infection is the presence of symptoms. Depending on the type of infection, the symptoms produced can range from the inconvenience of a cold to those that are life threatening.
Until the middle of the twentieth century, infections posed a serious problem even in developed countries. Throughout recorded history, infections often killed millions of people in epidemics of diseases like bubonic plague and typhoid fever. Even today, infections continue to cause more deaths during times of war and famine than does battle and starvation. Infections can sweep through a population quickly. For example, the acquired immunodeficiency syndrome (AIDS) has only been known for a little over three decades. Yet, AIDS is now the leading cause of death among African males.
Three factors are important in the control of an infection. These include identifying and eliminating the source of the infection, preventing the spread of the infection, and increasing the resistance of the host to the infecting microbe.
The hundreds of different infections that can occur in humans are caused by five major groups of microbes. These groups are the bacteria, a group made up of Rickettsiae, Coxiella, and Chlamydiae; viruses; fungi; protozoa; and worms known as Helminths. Infections from most of these organisms can be cured or made less severe using antibiotic drugs and anti-fungal medication. However, there is no cure for viral infections.
Most of the infections that humans acquire come from other people, animals or insects, and from nonliving objects that have infectious microbes adhering to them. Examples include the passage of a cold virus by kissing or sneezing, transfer of infectious viruses by dog or bat bites (i.e., rabies), use of contaminated needles to inject drugs (i.e., hepatitis B), unprotected sex with a contaminated partner (i.e., AIDS, syphilis). Infections also arise from drinking contaminated water or eating contaminated food.
Infections can become established when the immune system is not functioning properly because of disease, malnutrition, or treatment for another malady (i.e., chemotherapy for cancer). In these cases, microbes that would otherwise be easily defeated are able to proliferate, causing opportunistic infections.
Other infections arise because of a genetic condition in the host that predisposes the host to infection. One example is the persistent lung infections caused primarily by the bacterium Pseudomonas aeruginosa in some people who have cystic fibrosis. The fluid that accumulates in the lungs enables the bacteria to establish colonies that are resistant to treatment.
Still another route of infection is via the air. This route is especially relevant for bacterial spores, which are so small and light that they can float through the air and be inhaled. A prominent example is Bacillus anthracis, the cause of anthrax.
The concept of resistance to infection also applies to the host. As some bacteria are able resist host defenses and cause infection, so the host has several mechanisms of resistance. The first line of a host's defense is the various surfaces of the body. The skin, mucous membranes in the nose and throat, and tiny hairs in the nose that act to physically block invading organisms. The uppermost cells of the skin secrete chemicals that are lethal to bacteria such as Staphylococcus aureus, a bacterium that can cause skin infections. Microbes can also be washed away from body surfaces by tears, bleeding, and sweating. These are nonspecific mechanisms of resistance.
A host also has a specific defense response, namely the immune system. An invading microbe can be recognized as a foreigner and destroyed. This host resistance can be aided by vaccination, which in some cases provides a life long resistance to a particular organism.
The use of antibiotics was thought to be as powerful a deterrent to infection as vaccination. Indeed, when antibiotics were discovered in the middle of the twentieth century, many infections were presumed to have been defeated. However, this has proved not to be the case. The cause of the failure of some antibiotics is the ability of the target bacteria to become resistant to the drug. In the 1990s, this problem became especially evident, with the emergence of several types of infectious bacteria that are resistant to almost all antibiotics. Indeed, a strain of Staphylococcus aureus is resistant to all currently used antibiotics.
The development of resistance to antimicrobial agents such as antibiotics can have molecular origins. The membrane(s) of the bacteria may become structurally changed so as to make the passage of drugs across the membrane(s) difficult. Secondly, enzymes capable of degrading the antibiotic are produced. The overuse or inappropriate use of antibiotics (i.e., to treat a viral infection, even thought viruses are not affected by antibiotics) has contributed to the development of bacterial resistance, which can be genetically passed on to subsequent generations
The organization of the infecting microorganisms can also be a resistant factor. An example is the resistance that develops as a consequence of the surface growth of bacteria. In this mode of growth, which is known as a biofilm, the bacteria grow inside a sugary coating that is excreted by the surface adhering bacteria. Inside the coating the bacteria become almost dormant. The slow chemical activities of the bacteria, combined with the presence of the protective coating, makes biofilm bacteria extremely hardy. An example of the resistance of biofilm bacteria is that of Pseudomonas aeruginosa. Biofilms of this bacterium cause chronic lung infections in people afflicted with cystic fibrosis, and can grow on artificially implanted material (i.e., urinary catheters and heart pacemakers.)
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Centers for Disease Control. "National Center for Infectious Disease." [cited November 20, 2002] <http://www.cdc.gov/ncidod/>.