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【大英课堂资料】AIDs and HIV

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Human immunodeficiency virus (HIV) facts

·HIV is thevirus that causes HIV infection and the acquired immunodeficiency syndrome(AIDS).

·Anal orvaginalsexualintercourse and illicitinjectable drug use commonly transmit HIV. Infected mothers may also transmitHIV to their child duringpregnancyorbreastfeeding. Lesscommon routes of transmission include needle-stick injuries or exposure tocontaminated blood.

·The bloodsupply in the United States is tested for HIV before use, and statistics showthe risk of acquiring HIV infection from a transfusion is less than one in 1.5million.

·HIVattacks the immune system, especially cells known as CD-4 lymphocytes. Seriousimpairment of the CD-4 lymphocytes makes people susceptible to specificinfections and cancers.

·UntreatedHIV infected progresses through three stages, with stage three being AIDS.

·Health-careprofessionals diagnose HIV with tests that measure antibodies against the virusor measure the virus directly.

·Treatmentwith highly active antiretroviral therapy (HAART or ART) dramatically increaseslife expectancy although it does not cure HIV infection.

What is the human immunodeficiency virus?

The human immunodeficiency virus is thecause of HIV infection and the acquired immunodeficiency syndrome (AIDS). HIVbelongs to a family of organisms known as retroviruses. Once someone acquiresthe virus, it attaches to and enters human cells, especially cells known as CD4T-cells, macrophages, and dendritic cells. The virus contains RNA, which ittranscribes into DNA using an enzyme called reverse transcriptase. Theresulting DNA integrates into the human genome in the cell. In this way, thevirus fools the human genome into making more copies of the virus.

HIV may remain quiescent (latent) in thegenome or may be actively transcribed, causing the virus to replicate. HIV is aprolific virus and is able to create trillions of copies of itself in a shortperiod of time. During times of peak viral reproduction, even 1 milliliter ofblood can contain more than 1 million copies of the virus. Many of these copiesdiffer in small ways from the original virus and may be resistant to differentmedications. There are two types of HIV: HIV-1 and HIV-2. HIV-1 is the primarycause of HIV infection and AIDS in the world. HIV-2 is less common and lesseasily transmitted.

What is the history of HIV?

Although HIV infectionis only a recent discovery, scientists have shown that HIV-1 may have spread tohumans over 100 years ago. The likely source was primate-to-human transmissionthrough bites or blood exposure, with chimpanzees being the most likelycandidate for HIV-1 transmission to humans.

Isolated human casesexisted long before HIV infection came to the attention of the physicians. Itis not entirely clear what caused HIV to begin to spread more widely in themid-20th century. HIV was identified in tissue preserved from patients who diedas early as 1959. In the late 1960s or early 1970s, it is thought that HIVinfection spread to Haiti and then to the United States. Entering thehomosexual male community predominately, the virus began to spread among peoplewith multiple sexual partners. Over time, often a decade after infection,people with HIV infection began to get unusual infections, develop AIDS, anddie.

In1981, the Centers for Disease Control and Prevention first took note of acluster ofpneumoniacausesbyPneumocystis jirovecii(formerlyknown asPneumocystis carinii), which was a highly unusualpathogen to find in young men. Affected individuals were noted to have depletedcounts of a specific type of immune cell, known as the CD-4 T cell. Subsequentinvestigation revealed that homosexual men with multiple partners were mostlikely to have disease, suggesting that sexual activity was a key factor intransmission. Soon, scientists discovered the disease in people injectingillicit drugs, in heterosexuals with multiple sexual partners, in people whoreceived blood products, and in babies born to infected mothers. A globaleffort was undertaken to identify the virus, leading to the description of HIVas the likely cause of AIDS by Dr. Luc Montagnier in France and by Dr. RobertGallo in the United States. A test for HIV was soon developed and used todiagnose individuals and to ensure the safety of the blood supply.

What causes an HIV infection?

HIV is transmitted when infected materialsuch as blood or semen or other infected fluids gains access to a new host. Inthe United States, surveillance statistics show there are approximately 1.1million people living with HIV infection, but 18% are unaware that they areinfected. Approximately 50,000 people get infected with HIV each year. Althoughthe epidemic first was recognized in homosexual men, it has spread broadlythroughout the U.S. New infections affect people from all subpopulations,although some groups are more likely to be affected than others. Of the 47,500new infections in 2010, 63% were in men who had sex with men (MSN), 25% wereacquired through heterosexual contact, and 8% were from injection drug use. HIVinfection has hit the African-American community harder than other groups, with44% of new infections occurring in this population, compared to 31% in whitesand 21% in Hispanics. Over 600,000 Americans have died of HIV infection sincethe epidemic began.

Worldwide, there are approximately 34million people living with HIV, and there are 2.5 million new cases each year.Since HIV infection was recognized, there have been 30 million deaths as aresult of HIV infection.

What are the different stages of an HIV infection?

Untreated infection with HIV progressesover time and gradually impairs specific parts of the immune system, especiallyby destroying the white blood cells known as CD4 lymphocyte cells. Thisprogression is described as occurring in stages. All stages require laboratoryconfirmation of HIV infection.

There are multiple different stagingsystems. For example, the Centers for Disease Control and Prevention casedefinition uses a staging system based on how much damage has been done to theimmune system:

·Stage 1disease is theearliest phase. Stage 1 has no unusual infections or cancers or otherconditions that would be associated with AIDS. In other words, stage 1 diseasehas no AIDS-defining conditions (see below). Although blood testsare positive for HIV, the CD4 cell count is at least 500 cells per microliterof blood (or >29% of all lymphocytes).

·Stage 2disease occurs whenthe CD4 count is between 200-499 cells per microliter (14%-28% of alllymphocytes), but again there are no AIDS-defining conditions present.

·Stage 3disease is synonymouswith AIDS and is the most severe stage. There are two ways of diagnosing stage3 disease: either by CD4 counts below 200 cells per microliter (<14% oflymphocytes) or through documentation of an AIDS-defining condition.

Another way to conceptualize HIV isaccording to the characteristics or clinical manifestations: acute infection,clinical latency, or AIDS.

·Acuteinfection: Two tofour weeks after infection with HIV, the patient can experience an acuteillness, often described as the worstfluever. This is called acute retroviralsyndrome (ARS) or primary HIV infection, and it is caused by the body's naturalresponse to the HIV infection. Not all newly infected people develop ARS,however -- and it can take up to three months for it to appear. During thisperiod of infection, large amounts of virus are being produced. The virus usesCD4 cells to replicate and destroys them in the process. Because of this, theCD4 count can fall rapidly. Eventually, the immune response will begin to bringthe level of virus in the body back down to a level called a viral setpoint, which is a relatively stable level of virus in the body. At thispoint, the CD4 count begins to increase, but it may not return to pre-infectionlevels. The human immune response suppresses the virus but does not eliminateit from the body.

·Clinicallatency: After the acutestage of HIV infection, the disease moves into a stage called clinical latency.This period is sometimes called asymptomatic HIV infection or chronic HIVinfection. During this phase, HIV reproduces at very low levels, although it isstill active. In this state, infected people may be able to maintain anundetectable viral load and a healthy CD4 cell count without the use ofmedication for a time. There are usually few if any symptoms. This period canlast up to eight years or longer. However, some people progress through thisphase faster than others. It is important to remember that people are stillable to transmit HIV to others during this phase. Toward the middle and end ofthis period, the viral load begins to rise and the CD4 cell count begins todrop. As this happens, infected people may begin to have constitutionalsymptoms such asfatigueand other nonspecificsymptoms.

·AIDS: As the number of CD4 cells falls below200 cells per cubic millimeter of blood (200 cells/cubic milliliter), peoplewill be diagnosed as having AIDS. Normal CD4 counts are between 500 and 1,600cells per cubic milliliter. This is the stage of infection that occurs when theimmune system is badly damaged and patients become vulnerable to opportunisticinfections. Without treatment, people who are diagnosed with AIDS typicallysurvive about three years. Once someone has a dangerous opportunisticinfection, life expectancy falls to about one year.

AIDS-defining conditions in an HIV-infectedperson include the following:

·Candidiasisof bronchi, trachea, lungs, or esophagus

·Cervical cancer,invasive

·Disseminatedor extrapulmonary coccidioidomycosis orCryptococcus

·Chronicintestinalcryptosporidiosisor isosporiasis

·Cytomegalovirusdisease of the retinaor an unusual site (other than liver, spleen, or nodes)


·Herpessimplex that does notheal or that occurs in the lungs or esophagus

·Histoplasmosisthat is disseminated or extrapulmonary


·Lymphoidinterstitial pneumonia and/or pulmonary lymphoid hyperplasia*

·Selectedlymphomas including Burkitt's, immunoblastic, or arising in the brain

·Disseminatedor extrapulmonaryMycobacterium avium-intracellularecomplex orMycobacterium kansasii, or other species of mycobacterium



·Recurrentbacterial pneumonia

·Progressivemultifocal leukoencephalopathy

·Recurrentor multiple bacterial infections


·Toxoplasmosisof brain

·Wastingsyndrome associated with HIV infection

What are risk factors for an HIV infection?

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The most common modes of transmission inthe United States are via

·anal orvaginal sexual intercourse

·use ofillicit, injectable drugs where needles/syringes/materials are contaminatedthrough sharing them with an HIV-infected person.

Thus, the main risk factors for HIVinfection are having nonmonogamous sexual intercourse, having unprotected sex(not usingcondoms), or the useof injectable illicit drugs. Transmission through oral sex is less common butis still possible.

There is also a risk of transmission frominfected mother to unborn child during pregnancy or delivery. Appropriate use ofanti-HIV medications can significantly reduce this risk (see below) and hasdramatically reduced the incidence of HIV in newborns in the United States.There is also some risk in the perinatal period because breast milk may harborHIV, and infected mothers in the United States are counseled not to breastfeed.In developing countries, alternatives to breast milk may be lacking andrecommendations may differ. Currently, there are approximately 200 new HIVcases per year in the U.S. in children younger than 13 years of age, and most(75%) were infected after birth.

Other less common modes of transmissioninclude accidental needle stick or sharps injuries in health-care workers andsplashes of contaminated material onto mucous membranes or nonintact skin. Riskfactors include unsafe practices for handling used needles or sharps,inappropriate disposal of needles/sharps, and inadequate use of personalprotective measures such as gloves, masks, and eye protection when there is achance of splashing.

The blood supply in the United States istested for HIV before use and the risk of acquiring HIV infection from atransfusion is less than one in 1.5 million.

Casual contact (shaking hands, sneezing,coughing, sharing drinking glasses, sharing a toilet, exposure to saliva fromsocial kissing, etc.) does not pose a threat for HIV infection.

What are HIV infection symptoms and signs?

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As described above, although some peoplehave nosymptomsin the early weeksafter acquiring HIV, between one-third and one-half will experience symptoms offatigue, achiness, sore throat, enlarged lymph nodes, andloss of appetite.Mouth symptoms might includethrushormouth sores.Fever,neck stiffness,headache, andrashmay occur. Symptoms inwomen may include recurrent vaginal yeast infections. This acute retroviralillness (ARS) usually starts one to six weeks after infection and lastsapproximately two weeks. Some people experience ARS as long as three monthsafter initial infection. During this time, the blood is teeming with HIV andthe CD4 lymphocyte count is reduced, creating susceptibility to unusualinfections. Antibodies against the virus are beginning to form, the viral setpoint is established, and the infected person becomes asymptomatic, althoughsome may have persistent moderately enlarged lymph nodes. As disease advances,other conditions may appear. Although not specific to HIV, symptoms in womenmay include recurrent vaginal yeast infections, and symptoms in men who havereceptive anal sex may include severe or recurrent herpes infections. Mouthproblems might include thrush or oral hairyleukoplakia, whichis due to infection with the Epstein-Barr virus.

If patients are not treated, they progressto stage 3 in approximately 10 years. Patients in stage 3 have immune systemsthat are so impaired that they create susceptibility to unusual infections orcancers. These AIDS-defining conditions are listed above. Symptoms depend onthe type of infection orcancerthat is acquired. Forexample, patients with pneumonia may have shortness of breath andcoughorwheezing. Occasionally, HIV may cause anAIDS-defining condition directly through intense infection of the brain, whichcausesconfusionand encephalopathy.

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