We need to recognize that infectious “disease” is an “excessive immune response,” which is a type of inflammation caused by the invasion of pathogens, although you may be surprised to hear this. The immune system is an integrated force to keep the immune balance leading to fight microbial infections, which is composed of, for example, blood cells and cytokines and bone marrow. I often hear the word “immune power,” but the term “resistance power” has already been used to express the power to fight infections.

The term “immune system” refers to physiological conditions under a certain balanced state of harmony among many cells and cytokines. Therefore, once this balance is broken, a condition called a “cytokine storm,” which has often been heard these days, occurs in case of COVID-19 infection. When there are no symptoms, immunity is well-balanced without developing inflammation or an excessive immune response, even though the host is infected. In other words, it is determined by the relative balance of strength between invasive pathogens and “innate resistance,” whether or not inflammation is suppressed.

What are the differences between microorganisms and pathogens? Answering this question may help you to better understand infectious diseases. There is little difference between the two. There are countless microorganisms in the intestine called “intestinal flora.” Numerous indigenous bacteria actually exist on the skin surface, in the nose, the ears, the respiratory tract, and the vulva of females. However, the biological defense system of our body, which triggers immune responses, develops and acquires strength through exposure to antigen stimulation. These indigenous bacteria turn into pathogens when the immune system of the host is weak. In a word, the nature of their existence is relative. Disinfection, sterilization and pasteurization methods were originally not developed to fight against these indigenous bacteria until the 19th century. Many pregnant women died due to puerperal fever in the delivery rooms of hospitals. Many of these cases were caused by indigenous bacteria, because medicine at that time was just before the dawn of the brilliant modern medicine.

Let me roughly classify modern infectious diseases into several categories:

Infections that occur in daily life:

• Respiratory tract infections: droplet infections (including COVID-19)
• Airborne infections (influenza virus, cold syndromes)
• Oral infections: ingestion of spoiled foods
• Percutaneous infections (mosquito infections, like dengue fever)Infections at medical facilities:

Infections at medical facilities:

• Infections caused by insufficient sterilization or disinfection with ultraviolet irradiation (DNA, RNA destruction) at surgical facilities.
• Infections caused by needlestick injury by healthcare professionals
• STDs (sexually transmitted diseases) (HIV-AIDS virus, herpes simplex, chlamydia)
• Mother-to-child transmitted infections (hepatitis B virus)

Once the cause of the infection is detected, we have many defensive measures to take. In daily life today, various kinds of defensive measures are already available for human beings against infections other than COVID-19. Unfortunately, however, vaccines for COVID-19 are at last going to be developed, hopefully. Furthermore, medicines which have dramatic effects to this disease have not yet appeared.

Antibiotics have already exerted revolutionary effects on bacterial infections. In addition, we have already been provided with vaccines and antiviral drugs for viral infections, including smallpox vaccines. Various kinds of treatment medicines for HIV are available at present, and progress in its treatment is remarkable. Anti-hepatitis virus medicine is now administered orally, which was almost unimaginable several years ago. Until the end of the 20^th century, hepatitis patients were required to take several kinds of medicine by injection. We are now provided with a wide variety of treatment measures. Nevertheless, it is still important for us to take care of our immune system.

As I mentioned above, it has been reported that there are several infection routes for various pathogens and viruses. Therefore, careful care of these parts of the body should help to prevent infectious diseases. Next, I would like to consider preventive measures against trans-gastrointestinal tract and trans-respiratory infections.

We need to carefully take care of the oral and nasal cavities, since these areas are major channels of infection. I remember that once, before the onset of the COVID-19 pandemic, we rarely paid attention to the idea of keeping a space between people. Research on droplets is progressing, including, for example, droplets generated by speaking, their diffusion and drift pathway in the air, and the need for adequate air circulation in closed spaces. Some research has reported on the diffusion and drift status of droplets in environments like theaters, live music clubs, and other live entertainment settings. In these facilities, the method of ensuring a large space between the stage and the front row seats must be carefully examined. Next, I would like to mention measures to prevent infection through the oral cavity and nasal cavity.

1. Masks and “Avoid the Three Cs”

A mask that covers the nose and mouth during conversations provides us with the first line of defense. It is an effective preventive measure not only for COVID-19, but also for influenza virus. The material of a mask is very important. I used to underestimate the effect of a mask as a defensive measure for virus infections, thinking that the effect of a mask is just warming and moisturizing our throat, because the mesh of a mask is so porous that it can easily let viruses pass through. However, when I thought about this matter again, I realized the fact that the virus isn’t solely diffused out of the oral cavity or the respiratory tract in isolation, but within droplets. I was surprised, since this fact had been already proven and reported in the latest study on COVID-19 by the research institute of the WHO. In this study, the size of the outlets containing virus was measured in microns (1/1000,000 of a meter). How droplets are diffused and drift in the air was also analyzed.

2. Hand Washing with Soap

I think that many people usually wash their hands only with running water, but this is not sufficient “hand washing.” My father was an ophthalmologist, and he strictly disciplined me to carefully wash my hands with soap since my childhood. I still remember that when I was an elementary school student, my friends were severely instructed by myself. “Wash your hands with soap!” since they were washing their hands only with running water. Most microorganisms and viruses contain lipids in their cell membranes or surface coating. Soap may destroy pathogens, although not completely. In order to completely destroy them, sterilization with high concentration ethanol is an important measure to destruct their envelop structure with lipids.

3. Gargling

Extensive gargling into the lower throat is an effective defense measure for infectious diseases. First of all, gargle with water several times, secondly use isodine, a type of mouthwash containing povidone-iodine. It should be noted that you swallow the last drop. It washes and sterilizes the mucosa and surface of the throat extensively in the lower region.

COVID- 19 is reported to first infect the olfactory nerve. The olfactory nerve extends from the base of the brain into the upper part of the nasal cavity, and receptors of the virus were shown to be expressed around the olfactory epithelium. These findings are consistent with the symptoms, including olfactory and taste disorders, which develop in the early stages of infection. It could be helpful to wash the nasal cavity with physiological saline water.

4.  Immunity Care

Please let me mention again that the basic role of the immune mechanism is “self and non-self-awareness,” “self-tolerance,” and “rejection of non-self,” which are fundamental phenomena. In a healthy condition, several billion cells that support immunity are well balanced, and their harmony is maintained.

Immune responses are triggered when the balance between innate immunity and adaptive immunity is disturbed. As the strength of the inflammatory response is enhanced, its scale is determined by immune checkpoints. After inflammatory reactions subside from the sequence of these activities, the immune system returns to its former quiet condition. This system is controlled by the natural immune system, consisting of “monocytes-macrophages-dendritic cells,” which is like a flag waver. Signals from this flag waver maintain the well-balanced harmony of immunity through interactions with T lymphocytes, B lymphocytes, and NK cells.

The understanding of COVID-19 has gradually advanced, and several important points concerning immunity can be reported as follows:

1. Lower zinc levels are observed in COVID-19 patients with severe symptoms. Foods high in zinc include oysters, liver, and natto.

2. T lymphocytes are stronger in females. Females have more inflammatory cytokines.

3. Irisin, a recently discovered hormone, is produced in muscles. This hormone is effective in treating infections.

4. Cytotoxic T lymphocytes in elderly males respond weakly against COVID-19.

5. Vitamin D intake is correlated with improvement of symptoms.

6. Blood IL-17A, which is high in children, is effective in infection prevention. It leads the idea that plasma from the youth help the aged to support and protect against COVID-19 by plasma transfusion therapy.

Many other findings have also been reported. I expect that this information will contribute to the progress of immune prevention and treatment for infectious disease. I am completing this series of six essays. I hope this information will support your scientific wisdom in your daily life to prevent infection.