3. HIV Epidemic
The vast majority of people living with HIV live in low - and middle-income countries. Africa has the world's largest number of people living with HIV/AIDS, and southern Africa is even worse. South and Southeast Asia are the second worst affected areas after Sub-Saharan Africa. The development of HIV risk environments  has been shaped by social-structural, economic and political factors specific to each context.
Figure 2 New HIV infections (all ages)-by region
HIV outbreak: By the end of 2016, about 36.7 million people worldwide were living with HIV. Of these, 2.1 million are children (under 15 years of age). In 2017, 19.6 million people were living with HIV in eastern and southern Africa, 6.1 million in western and central Africa, 5.2 million in Asia and the Pacific, and 2.2 million in western and central Europe and North America, 1.8 million people were living with HIV in Latin America, 1.4 million people were living with HIV in Eastern Europe and central Asia, 2.2 million people were living with HIV in Middle East and North Africa.
Figure 3 New trends in HIV infection in 2010-2017
AIDS breakthrough: Since 2010, the percentage change in AIDS-related deaths has fallen by 34%, the percentage change in the number of new HIV infections has fallen by 18%.
Figure 4 Trend of new HIV infections and AIDS-related deaths
More specific data can be found in the UNAIDS: http://aidsinfo.unaids.org./
At present, this international response to AIDS is unprecedented because resources are more committed than any other health cause . This is called the AIDS exceptionalism.
5. Causes of AIDS
5.1 Source of Infection
HIV-infected persons are the source of infection. HIV has been isolated from blood, semen, vaginal secretions, milk, etc.
The ability of HIV to survive in vitro is extremely poor, it is not resistant to high temperatures. So shaking hands, hugging, kissing, swimming, mosquito bites, sharing cutlery, coughing or sneezing, daily contact, etc. will not spread.
5.2 HIV Transmission
The following are the three main modes of communication :
HIV is present in the semen and vaginal secretions of infected people. Sexual behavior can easily cause minor skin mucosa damage, and the virus can be infected through the damaged area into the blood.
The human body is exposed to blood or blood products containing HIV, intravenous drug use, and transplant tissues and organs of infected patients are at risk of contracting AIDS.
Women infected with HIV can pass the virus to the fetus during pregnancy and childbirth. The infected woman can also pass the virus to the child who is breastfeeding through breastfeeding.
Figure 6 Transmission of AIDS
Figure 7 No HIV transmission
5.3 Pathogenic Mechanism
It is well known that AIDS is an Immunodeficiency disease caused by the Human Immunodeficiency Virus (HIV). HIV selectively invades CD4 molecules, mainly T4 lymphocytes, monocyte macrophages, dendritic cells, and the like.
The CD4 molecule on the cell surface is the HIV receptor. After the HIV membrane protein gp120 binds to the CD4 on the cell membrane. The conformation of gp120 changes, causing gp41 to be exposed. Meanwhile, gp120-CD4 binds to the chemokines CXCR4 or CXCR5 on the surface of the target cells to form the cd4-gp120-cxcr4 /CXCR5 tri-molecular complex. Gp41 uses its hydrophobic properties to mediate the fusion of viral cysts and cell membranes. Eventually the cells are destroyed. Over time, HIV destroys so many of these cells that the body cannot fight infection and disease, making people more susceptible to other infections or infection-related cancers.
5.4 The Stage of HIV Infection
The three stages of HIV infection are: Acute HIV Infection Stage; Clinical Latency Stage; AIDS (Acquired Immunodeficiency Syndrome).
Acute HIV Infection Stage
Many (but not all) people develop flu-like symptoms within 2 to 4 weeks of infection. HIV symptoms include fever, gland swelling, sore throat, AIDS rash, muscle and joint pain, and headache in this stage.
Clinical Latency Stage
The incubation period refers to the period in which the virus survives or develops in the human body without causing symptoms. During the clinical latency stage, standard laboratory tests fail to detect the virus. But people in this period can still transmit HIV to others. The average clinical latency stage was 10 years.
AIDS (Acquired Immunodeficiency Syndrome)
AIDS is the most severe stage of HIV infection. The immune system of AIDS patients is seriously damaged, and they will suffer from more and more serious diseases, called opportunistic infections. When your CD4 cell count is less than 200 cells per cubic millimeter of blood (200 cells/mm3), you are considered to have developed AIDS. (In populations with a healthy immune system, the number of CD4 cells is between 500 and 1600/mm3).
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