Infectious disease researchers explain the process by which people become infected with bacteria and viruses, and what are the effective measures to prevent infection?
Pathogens such as viruses and bacteria repeatedly invade the human body in various forms on a daily basis, but the human body is equipped with a defense system called immunity, so it does not cause illness. However, if a large number of pathogens invade the body at once, the body's defenses cannot keep up and the disease develops. Tara C. Smith, who studies infectious diseases, explains how to become less likely to get sick based on the process of disease onset.
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In order for a pathogen to cause a 'sick' state in the human body, it must overcome barriers such as the skin, mucus, cilia, and stomach acid to enter the body. Some bacteria and parasites can begin the process of reproduction anywhere in the body, while other bacteria and viruses cannot begin the process of reproduction without entering cells. Additionally, while in the human body, they are constantly attacked by the immune system. When pathogens break through these barriers and begin to multiply, they become a disease.
Pathogens invade the human body on a daily basis, but the number of invading pathogens is usually too small for the immune system to overcome. The more pathogens there are, the more likely it is that the immune system will be overcome, and the more likely you are to get sick. ``How many pathogens need to enter the body before a disease develops?'' varies depending on the pathogen, and is expressed by the ``
Generally speaking, a person will not become ill unless the number of pathogenic bacteria increases considerably, but there are some pathogenic bacteria, such as norovirus, that can cause illness with extremely small amounts of pathogenic bacteria. The minimum amount of virus for norovirus to develop is `` 18 viruses ,'' and it can survive outside the body for several days, so if an infected person spreads the virus once, it will remain in that place for several days. There is a risk of spreading the infection to everyone who comes into contact with it.
It is not yet clear why the minimum amount of bacteria that causes symptoms differs depending on the pathogen, but as of September 2023, the prevailing idea is that ``pathogens that directly attack cells have a small minimum amount of bacteria that causes symptoms.'' It is thought that the more indirect the means of attacking cells, such as by secreting proteins that harm cells, the more bacteria will be needed to cause symptoms, as the attack will be diluted over time and space. .
In research to determine the minimum amount of bacteria that can cause symptoms, it is ethically difficult to administer pathogens to humans, as there is a risk of significantly worsening the disease or causing long-term complications. etc. are used. Other methods exist that do not directly administer the pathogen, but rather observe how long it takes for people who have daily contact with an infected person to become ill.
The route of infection is also important for the actual onset of the disease. In particular, if the virus directly enters the bloodstream, it can bypass many of the human body's defense mechanisms, so it takes a much smaller amount of bacteria to cause illness than if it enters through the mouth or lungs. When comparing the risk of HIV transmission, blood transfusions and injections are far more dangerous than sexual intercourse.
Apart from the 'minimum onset viral load,' there is also the concept of ' viral load .' While the minimum viral load indicates the amount of virus required to cause infection, the viral load is an index that indicates how much virus exists in a certain area such as the body, and is used for treatment of HIV/AIDS, etc. It is used when managing.
Various studies have been conducted in the four years since the emergence of the new coronavirus that causes COVID-19, and the results of these studies indicate that the ``minimum viral load'' is as small as 100 to 400 PFU ( plaque forming units ). It has been suggested that this is one of the reasons why the new coronavirus is so contagious. On the other hand, it has been shown that patients infected with the new coronavirus tend to have a high viral load, expelling up to 800 pieces of viral RNA per minute for eight days after the onset of symptoms. It is not possible to know how much of the virus was alive from the viral RNA, but if half of this viral RNA was ``live new coronavirus'', the infection could spread with just one minute of close contact. is.
When a previously infected pathogen invades, antibodies produced during the previous infection bind to the pathogen, preventing it from attaching to cells or marking it for attack by neutrophils , a type of white blood cell. Masu. Furthermore, even after the cells have been invaded, attacks by memory T cells begin to take place. In this way, when the same pathogen invades multiple times, various additional defense mechanisms come into play, increasing the minimum amount of bacteria that can cause symptoms and making it harder to develop symptoms. In addition to actually getting infected, vaccination can have the same effect as infection, making the body less susceptible to developing the disease.
In order to avoid infectious diseases, it is important to reduce the concentration of pathogens and the contact time. In addition to reducing the concentration of pathogens by wearing masks, providing ventilation, and maintaining social distance, vaccination can increase the minimum viral load and make it difficult for people to become infected. You can reduce the risk of developing the disease. In addition, research on the new coronavirus has shown that vaccinated people tend to reduce the amount of virus released, so it can be said that vaccination is important to prevent the spread of infection to those around you. is.
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