Microorganisms are everywhere, such as soil, fresh water, seawater, even under the sea floor and in the air. Every day we eat, drink and inhale microorganisms. However, although microorganisms seem to be everywhere, human bodies are rarely invaded by them and infected after reproduction. Even if they cause infection, sometimes the infection is very mild and will not cause any symptoms.
Relatively few microorganisms can directly cause disease. Most microorganisms exist in human skin, mouth, respiratory tract, intestine and reproductive tract (especially vagina). Whether microorganisms are harmless companions of the host or invade the body to cause diseases depends on the characteristics of microorganisms themselves and the defense ability of human body.
★ Resident flora
A healthy person lives in harmony with the normal microbial community fixed in a special part of his body. This normal flora occupying a specific part of the human body is called permanent flora. Not only do they not cause diseases, but resident bacteria usually protect people from pathogenic bacteria. If this coordination is broken, resident bacteria can try to rebuild their position and colonize. Microorganisms colonize the host for hours or weeks, but cannot permanently settle down, which is called temporary bacteria.
Environmental factors, such as diet, environmental conditions, air pollution and sanitary habits, can affect some strains to become permanent bacteria in the human body. For example, lactobacillus is a common resident bacterium in the intestines of people who eat a lot of dairy products, while Haemophilus influenzae is usually colonized in the airways of patients with chronic obstructive pulmonary disease.
In some cases, some microorganisms as permanent bacteria can also cause diseases. For example, Streptococcus pyogenes can live in the throat without causing harm, but when the body's defense ability is weakened or Streptococcus is a special strain, it will cause Streptococcus angina (throat infection). Similarly, due to the destruction of the host's defense barrier, other resident bacteria can also invade the body and cause diseases. For example, patients with colon cancer are vulnerable to bacteria that usually live in the intestine. These microorganisms can spread through blood and infect heart valves. Exposure to large doses of radiation can also cause these microorganisms to invade the body and cause systemic infection.
★ How did the infection happen?
Most infectious diseases are caused by microorganisms that invade and reproduce. The invasion of most microorganisms begins with attachment to human cells. Adhesion is a very special process, including the close connection between human cells and microorganisms. Whether microorganisms stay near the invasion site or spread far away depends on whether they produce toxins, enzymes or other substances.
Some microorganisms that invade the body will produce toxins, which are toxic to nearby or distant cells, and most of the toxin components will specifically bind with some molecules on specific cells (target cells), thus causing diseases. Diseases caused by this toxin include tetanus, toxic shock syndrome and cholera. A few infectious diseases are caused by toxins produced by microorganisms in vitro, and food poisoning caused by staphylococcus is a good example.
Several relationships between microorganisms and organisms;
* * * Life: Microorganisms and organisms live under mutually beneficial conditions;
* * * Habitat: Microorganisms and organisms coexist and do not harm each other;
Parasitism: Microorganisms benefit in this way, while the body suffers. Bacteria and fungi in most microorganisms have a relationship of * * * and * * *.
After microbial invasion, it is necessary to breed to cause infection. Then one of the following three situations may occur. First of all, microorganisms can constantly reproduce, defeat the body's defense system, cause enough damage, and even lead to death; Secondly, it can reach a state of balance and lead to chronic infection. In this struggle, neither the microorganism nor the patient won. Finally, with or without medication, the body can destroy invading microorganisms. At this time, the body can rebuild health and often provide a lasting immune state for another infection caused by the same microorganism.
Many pathogenic microorganisms have the characteristics of increasing disease severity (virulence) and tolerating the body's defense mechanism. For example, some bacteria produce enzymes that destroy tissues, which makes infections spread faster.
Some microorganisms have methods to block human defense mechanism, such as some microorganisms can interfere with antibody production or specifically attack the development of one of their white blood cells, T cells. Other microorganisms have shells (cysts) that can tolerate the digestion of white blood cells. Cryptococcus can form a thick capsule after invading the lungs. This is because this thick coating is formed in CO2 environment, and the CO2 in the lungs is more than that in the soil where fungi normally grow. In this way, when the lungs are infected by Cryptococcus, the body's defense mechanism cannot play a role. Some bacteria can tolerate the decomposition (dissolution) of certain substances in blood circulation, and some microorganisms can even produce chemicals that resist the effects of antibiotics.
★ Influence of infection on the body
Some infections can cause changes in blood, heart, lung, brain, kidney, liver and intestine. Doctors can determine whether there is an infection by determining these changes.
★ Changes in blood
As part of the body's defense against infection, the number of white blood cells is increasing. This leukocytosis can occur within a few hours after infection, which is the result of the release of white blood cells from stored bone marrow. Neutrophils in white blood cells increase first. If the infection persists, monocytes and other kinds of white blood cells will also increase. In other white blood cells, such as eosinophils, the increase is mainly caused by allergic reaction and parasite invasion, but it is usually not high in bacterial infection.
In some infections, such as typhoid fever, leukopenia is actually reduced. Leukopenia may also occur when bone marrow is obviously inhibited and enough white blood cells cannot be produced quickly to replace the white blood cells lost to overcome infection.
Infection caused by medical operation technology
It is generally believed that infection always occurs when microorganisms invade the human body and attach to special cells. However, microorganisms can also attach to medical devices placed in the human body, such as catheters, artificial joints, artificial heart valves, etc., and enter the human body for colonization with medical operation technology, and cause infection and spread with medical devices placed in the human body.
Anemia may occur in bleeding caused by infection, destruction of red blood cells or bone marrow suppression. Severe infection can lead to massive intravascular congestion, which is called disseminated intravascular coagulation. The best way to correct this situation is to treat the underlying diseases, especially the infections here. When there is only thrombocytopenia and no other changes, it may also come from infection (indicating a basic infection).
★ Changes of heart, lungs and brain
Possible cardiac changes during infection include increased heart rate and increased or decreased cardiac output. Although most infections have a high pulse rate, some infections, such as typhoid fever, have a relatively low pulse rate compared with the severity of their fever. Blood pressure will drop. In systemic infection, the extensive expansion of blood vessels will lead to a serious drop in blood pressure (septic shock).
Infection and fever usually cause the body to breathe faster (increase breathing frequency). This means that more carbon dioxide is transported from the blood and exhaled by the lungs, making the blood more acidic. Lung hardness may increase, which will affect breathing and lead to acute respiratory distress syndrome. The respiratory muscles in the chest are also prone to fatigue.
No matter whether microorganisms directly invade the brain or not, brain dysfunction may occur in severe infection, especially in the elderly, and high fever can also lead to convulsions.
★ Changes of kidney, liver and intestine
Renal changes can range from the loss of trace protein in urine to acute renal failure. Renal diseases can be caused by weakened cardiac function, including decreased blood pressure or direct damage to the kidney by microorganisms.
Many infections can change liver function. Although microorganisms do not directly attack the liver. The most common symptom is jaundice caused by bile reflux (cholestatic jaundice). Jaundice is a worrying sign when it is caused by infection.
Severe infection can cause stress ulcer in upper digestive tract, leading to bleeding. Usually, the amount of bleeding is not large, but a few people can also have massive bleeding.
★ The human body's defense against infection
The human body's defense system against infection includes: natural barriers such as skin, non-specific mechanisms such as some kinds of white blood cells and fever, and specific mechanisms such as antibodies. Usually, if a microorganism can cross the body's natural barrier, it will be destroyed by non-specific and specific defense mechanisms before its reproduction.
★ Natural barrier
Generally speaking, skin can protect the body from many microorganisms unless its integrity is destroyed. Such as trauma, insect bites or burns. There are exceptions, such as human papillomavirus can directly infect the skin to form warts.
Other effective natural barriers are mucous membranes, such as the superficial mucosa of respiratory tract and intestine. Typical mucous membrane is covered with a layer of secretion, which can resist microorganisms. For example, the mucous membrane of the eyes is soaked in tears, which contain an enzyme called lysozyme, which can dissolve bacteria and protect the eyes from infection.
Respiratory mucosa can effectively filter out particles in inhaled air. The mucosa covered on the curved wall of the nasal cavity helps to remove most of the inhaled foreign bodies. If a microorganism reaches the lower respiratory tract, the fine hair-like structure (cilia) covering the mucosal surface will expel it from the lungs, and coughing will further help remove these microorganisms.
The gastrointestinal tract also has a series of effective barriers, including gastric acid, and the antibacterial activities of pancreatic enzymes, bile and intestinal secretions. Intestinal contraction (peristalsis) and normal shedding of intestinal epithelial cells are helpful to eliminate harmful microorganisms.
The urogenital tract of human body is protected by a certain length of urethra (about 20cm long). Because of this protective mechanism, bacteria rarely enter the urethra unless they are inadvertently brought in by surgical instruments. Women are protected by the acidic environment of vagina, and irrigation such as bladder emptying is another protective mechanism for both men and women.
People with impaired body defense mechanisms are more susceptible to infection. For example, people without stomach acid are particularly susceptible to tuberculosis and salmonella. It is also important to keep the balance between the resident bacteria in the intestine. Sometimes, the human body ingests certain antibiotics due to an infection in a certain place, which will upset the balance among resident bacteria and lead to an increase in the number of pathogenic bacteria.
★ Nonspecific defense mechanism
Any kind of injury, including bacterial invasion, will cause inflammatory reaction. To some extent, inflammation is a direct defense mechanism to deal with the injured and infected sites. Through the inflammatory process, the blood supply increases, white blood cells enter the inflammatory site more easily through blood vessels, and the number of white blood cells in the blood stream also increases. Bone marrow releases a large number of white blood cells from storage, making them new members.
In this case, the first white blood cells are neutrophils, which begin to devour invading microorganisms and try to limit the infection to a small area. If the infection persists, another type of white blood cells (phagocytic monocytes) will increase in number. However, these nonspecific defense mechanisms will be overwhelmed (exceeded) by a large number of microorganisms or other factors that reduce the body's defense ability, such as air pollution (including smoking).
★ have a fever
Fever means that the body temperature rises above 38℃ (oral surface measurement). Fever is actually a protective response to infection and injury. The rise of body temperature can strengthen the body's defense mechanism and only cause relatively little discomfort.
Under normal circumstances, the body temperature fluctuates up and down every day, with the lowest at 6 o'clock in the morning and the highest at 4 ~ 6 o'clock in the afternoon. Although the normal body temperature is 37℃, it can actually reach 37.2℃ at 6 am and 37.6℃ at 4 pm.
The hypothalamus of the brain controls the body temperature, and the fever actually comes from the regulation of the hypothalamic thermostat. By moving blood from the skin surface to the inside of the body (shunting), the human body reduces heat loss and raises the body temperature to a new and higher constant temperature level. Tremor caused by muscle contraction will increase the production of heat. The body tries to conserve and generate heat until the blood flow makes the hypothalamus reach a new and higher body temperature. In this way, the body temperature will be maintained in a normal way. Later, when the body temperature is adjusted to a normal level, the body will eliminate excessive heat by sweating and letting blood flow to the skin. When the temperature drops, there will be chills.
Fever can peak every day and then return to normal, or it can be the opposite. At this time, although the body temperature fluctuates, it cannot be reduced to normal. Some people, such as alcoholics, the elderly or children, also have hypothermia when they are seriously infected.
The substance that causes fever is called pyrogen, which can come from inside or outside the body. As a representative of pyrogen in vitro, microorganisms and their products are called toxins. The pyrogen from the body is actually to stimulate the body to release its own pyrogen. The pyrogen formed in the body is usually caused by a white blood cell called monocyte. However, infection is not the only cause of fever, and all inflammatory processes, cancer or allergic reactions can also cause fever.
★ Determination of the cause of fever
Usually there is a definite cause of fever, such as flu or pneumonia. But sometimes the causes of fever are complicated, such as endocarditis (bacterial endocarditis). When someone has a fever of at least 38℃ and no cause is found after comprehensive examination, doctors usually call it unexplained fever (see section 263). The potential causes of this fever include any disease that causes fever. But at least in adults, the most common diseases are infection, diseases caused by human tissues (autoimmune diseases) and an undiscovered tumor (especially leukemia and lymphoma).
To determine the cause, doctors can start by asking patients about their present and past symptoms and diseases, recent drug use, contact infection and recent travel. The type of fever is usually not helpful for diagnosis. But there are some exceptions, such as fever every other day and fever on the third day, which are typical manifestations of malaria.
Recent travel, especially to foreign countries, or contact with certain substances or animals, can provide clues to the cause of the disease. Coccidiosis is prevalent in the southwestern United States, while histoplasmosis is prevalent in Ohio and Mississippi River basins. People who drink contaminated water (or ice products made from contaminated water) may get typhoid fever. People who work in meat processing plants may get brucellosis.
After asking the above questions, the doctor made a careful physical examination to find the source of infection and evidence of the disease. Whether the physical examination is in the infirmary or the hospital depends on the patient's fever and the severity of his illness.
Blood test can be used to detect anti-microbial antibodies, directly culture microorganisms and determine the number of white blood cells. The increase of specific antibody level helps to identify invading microorganisms. Elevated white blood cells usually indicate infection. The classification and counting of white blood cells (the proportion of all kinds of white blood cells) can give further ideas. For example, neutropenia indicates acute bacterial infection, and acidic leukocytosis indicates parasitic infection, such as tapeworms or worms.
Ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) scanning are helpful for diagnosis. Labeled white blood cell scanning can be used to determine the site of infection or inflammation. When doing this experiment, inject a radioactive white blood cell into the human body, because the white blood cell is attracted to the infected site, and the injected white blood cell has been radiolabeled, and the infected site can be determined when scanning. If the test result is negative, the doctor needs to take a biopsy from the liver, bone marrow or other suspected parts. Biopsy specimens were examined and diagnosed by microscope.
★ Treat fever
Because of the potential benefits of fever, there is controversy about whether it is necessary to carry out routine treatment for fever. However, if a child has ever had febrile convulsions, fever should be treated. Adults with heart and lung diseases should also be treated similarly, because fever will increase oxygen consumption. For every 0.5℃ increase, oxygen consumption will increase by 7% (when the body temperature rises above 37℃).
Drugs used to reduce body temperature are called antipyretics. Acetaminophen and non-steroidal anti-inflammatory drugs, such as aspirin, are the most widely used and effective antipyretics. However, aspirin should not be used for children and adolescents with fever, because it will increase the risk of Reye's syndrome, which may be fatal.
★ Specific defense mechanism
Once infection occurs, the immune system is fully mobilized, and the immune system produces various substances that can attack invading microorganisms. For example, antibodies can be adsorbed on invading pathogens to help fix pathogens from spreading, and antibodies can also directly kill pathogens or make them more vulnerable to white blood cells. In addition, the immune system can provide killer T cells (and other types of white blood cells) to specifically attack invading microorganisms.
Anti-infective drugs, such as antibiotics, antifungal drugs or antiviral drugs, can help the body's natural defense system. Therefore, if the human immune system is seriously damaged, these drugs often lose their effect.