In this blog post, we will examine the basic principles of the immune system and the infectious diseases that have long threatened humanity, and explore how HIV damages the immune system, leading to AIDS.
The Terror of Infectious Diseases and the Discovery of the Immune System
Throughout humanity’s history spanning millions of years, infectious diseases have always been a source of fear. Whenever a disease emerged—one whose cause, let alone a cure, was unknown—and swept through village after village, humanity had to face the fear that it might be on the brink of extinction. Examples include the plague (the Black Death, an acute infectious disease caused by the Yersinia pestis bacterium) that ravaged the late Middle Ages and the influenza pandemic that struck Europe toward the end of World War I. Through the research of scientists such as the French scientist Louis Pasteur (1822–1895), the relationship between microorganisms and infectious diseases was elucidated, and as vaccines and preventive medicine advanced, humanity gradually became able to cope with the fear of infectious diseases. However, since infectious diseases like AIDS are directly related to the immune system, there is still no cure that can completely eliminate HIV infection. Nevertheless, effective antiretroviral therapy now allows for long-term management of HIV infection, and people receiving appropriate treatment can maintain a healthy daily life. Therefore, I will explore the immune system and AIDS, which is related to it.
AIDS stands for Acquired Immune Deficiency Syndrome. AIDS was first officially reported in 1981 and has since spread worldwide. As of the end of 2025, approximately 41 million people worldwide are living with HIV, and it is estimated that about 1.2 million people were newly infected with HIV in 2025 alone. In the same year, approximately 570,000 people died from HIV-related illnesses, and about 32.1 million people were receiving antiretroviral therapy. If we look closely at the term “AIDS,” it is classified not as a “disease” but as a “syndrome.”
How do innate and adaptive immune systems differ?
To understand the effects of AIDS on our bodies, we must first understand “immunity.” Our bodies are equipped with a defense system against foreign substances; this is called immunity, which is broadly divided into innate and adaptive immunity. Innate immunity, as the term suggests, is the immunity we possess from birth. For example, the skin prevents the invasion of harmful bacteria and viruses, and secretions such as tears contain an enzyme called lysozyme, which destroys certain bacteria. Furthermore, when bacteria penetrate the body, components of the innate immune system—such as complement proteins and macrophages—act to eliminate pathogens that are harmful to our bodies. Adaptive immunity refers to the defense system that responds specifically to new foreign substances when our body is exposed to them. Adaptive immunity is carried out by T lymphocytes and B lymphocytes—types of white blood cells. T lymphocytes help activate other parts of the immune system or eliminate infected cells, while B lymphocytes produce antibodies. Antibodies are immune proteins that recognize specific foreign substances that have entered the body and respond to them.
We can examine the differences between these two immune systems through a diagram illustrating the characteristics of innate and adaptive immunity. “Innate immunity” refers to the innate immune system, while “Adaptive immunity” refers to the adaptive immune system. Adaptive immunity works either by T lymphocytes directly eliminating infected cells or by antibodies—produced by B lymphocytes activated with the help of T lymphocytes—recognizing and eliminating pathogens. The major differences between innate and adaptive immunity lie in specificity and the presence or absence of memory. Specificity refers to the characteristic of adaptive immunity to recognize a specific substance and mount an effective response against it, in contrast to innate immunity, which responds broadly when a foreign substance enters the body. Furthermore, “memory” refers to the immunological characteristic whereby, after an immune response to a specific foreign substance has occurred, some T lymphocytes and B lymphocytes remain as memory cells, enabling a faster and stronger response when the same substance re-enters the body. This demonstrates that adaptive immunity plays a crucial role in immune responses to new substances.
How does HIV destroy the immune system?
AIDS is a syndrome associated with a state in which the immune system—particularly CD4-positive T lymphocytes—is damaged as HIV infection progresses, leading to a significant decline in immune function. The cause of AIDS is HIV, or the Human Immunodeficiency Virus, which is primarily transmitted through bodily fluids such as the blood, semen, vaginal secretions, and breast milk of infected individuals. Major routes of transmission include unprotected sexual contact, the sharing of contaminated syringes or needles, and mother-to-child transmission from an infected mother. HIV infects and damages CD4-positive T cells—a type of T lymphocyte—and as the infection progresses, the immune system’s function gradually weakens. In the early stages of HIV infection, symptoms similar to those of a cold—such as fever, fatigue, and sore throat—may appear, though in some cases, symptoms are barely noticeable. If left untreated, HIV infection can damage the immune system over a long period, leading to a significant decline in immune function. Ultimately, when the immune system becomes severely weakened, the body becomes vulnerable to infections and diseases that healthy people recover from relatively easily, and it may become difficult to fight off various opportunistic infections and illnesses.
Why is HIV so difficult to cure?
Why can’t AIDS be cured? The key lies in how HIV replicates. HIV uses its own genetic information to replicate inside host cells, and in the process, it uses reverse transcriptase to convert its RNA genetic information into DNA. Errors occur relatively frequently during HIV replication, which can lead to various mutations. This characteristic is one of the factors that makes it difficult to completely eliminate HIV with a single drug or a single immune response. If HIV mutates, its susceptibility to existing immune responses or specific drugs may change. Since the immune response to HIV relies on recognizing specific viral structures, the virus’s continuous mutation makes it difficult for the immune system to completely eliminate it. It is like a key and a lock: the structure must be recognized precisely, but HIV continuously alters that structure by generating various mutations during its replication process.
There is still no cure for AIDS. However, thanks to various research efforts and medical advances, effective prevention and treatment methods are now available. People living with HIV can maintain a healthy life by consistently receiving antiretroviral therapy (ART), which suppresses viral replication and protects the immune system. Although current ART cannot completely eliminate HIV, it can suppress the viral load in the blood to very low levels; people who maintain a viral load that is undetectable do not transmit HIV through sexual contact. It is estimated that by 2025, 32.1 million people—approximately 78% of all people living with HIV worldwide—will be receiving antiretroviral therapy. Moving forward, based on our research on AIDS, we must focus on developing solutions for new infectious diseases. The Ebola virus outbreak, for example, demonstrates just how critical it is to rapidly establish effective prevention and treatment methods when an infectious disease emerges. Whether viewed historically or in the current context, infectious diseases are a challenge we must continuously address. We must advance these solutions as much as possible to minimize the damage caused by infectious diseases.
