Acquired Immunodeficiency Syndrome AIDS Epidemiology Epidemiologic studies in the United States have identified five groups of adults at high risk for developing AIDS The case distribution in these groups is as follows Men who have sex with men account for more than 50 of the reported cases 1 This includes about 5 who are intravenous drug users as well 2 Transmission of HIV inthis category appears to be on the decline but in 2016 this remained the largest affected group constituting about 70 of new cases Heterosexual transmission 1 chiefly due to contact with members of other high risk groups e g intravenous drug users is responsible for approximately 20 of cases in the United States 2 Heterosexual transmission has declined modestly since 2011 in the United States 3 Globally however heterosexual transmission is by far the most common mode by which HIV is spread and as a result women are much more frequently infected outside of the United States 4 In sub Saharan Africa where the infection rate is estimated to be about 10 000 new cases every day more than one half of infected individuals are women 5 Heterosexual spread of the virus is occurring most rapidly in female sex workers and in women in long term marital or cohabitating relationships particularly among adolescents Intravenous drug users with no previous history of homosexuality are the next largest group representing about 20 of infected individuals HIV infection of the newborn 1 Children of HIV positive women are at risk for infection in utero at birth or through breast milk 2 Close to 2 of all AIDS cases occur in children and it is estimated that worldwide there are 1 7 million HIV infected individuals younger than 15 years of age Patients with hemophilia especially those who received large amounts of factor VIII or factor IX concentrates before 1985 make up about 0 5 of all cases Recipients of blood and blood components who are not hemophiliacs but who received transfusions of HIV infected whole blood or components e g platelets plasma account for about 1 of patients Organs obtained from HIV infected donors can also transmit the virus In approximately 5 of cases the risk factors cannot be determined 1 It should be apparent from the preceding discussion that transmission of HIV occurs under conditions that facilitate exchange of blood or body fluids containing the virus or virus infected cells 2 The three major routes of transmission are sexual contact parenteral inoculation and passage of the virus from infected mothers to their newborns Sexual transmission is the dominant mode of infection worldwide 1 accounting for more than 75 of all cases of HIV transmission 2 Because the majority of infected people in the United States are men who have sex with men most sexual transmission has occurred among homosexual men 3 The virus is carried in the semen and it enters the recipient s body through abrasions in rectal or oral mucosa or by direct contact with mucosal lining cells 4 Viral transmission occurs in two ways 1 direct inoculation into the blood vessels breached by trauma and 2 infection of DCs or CD4 cells within the mucosa 5 In addition to male to male and male to female transmission female to male transmission also occurs 6 Sexual transmission of HIV is enhanced by coexisting sexually transmitted diseases especially those associated with genital ulceration 7 In this regard syphilis chancroid and herpes are particularly important 8 Other sexually transmitted diseases including gonorrhea and chlamydia are also cofactors for HIV transmission perhaps because in these genital inflammatory states there is greater concentration of the virus and virus containing cells in genital fluids as a result of increased numbers of inflammatory cells in the semen Parenteral transmission of HIV 1 has occurred in three groups of individuals intravenous drug users hemophiliacs who received factor VIII and factor IX concentrates and random recipients of blood transfusion 2 Of these three intravenous drug users constitute by far the largest group 3 Transmission occurs by sharing of needles syringes and other paraphernalia contaminated with HIV containing blood 4 Transmission of HIV by transfusion of blood or blood products such as lyophilized factor VIII and factor IX concentrates has been virtually eliminated 5 This fortunate outcome resulted from increasing use of recombinant clotting factors and from three public health measures screening of donated blood and plasma for antibody to HIV stringent purity criteria for factor VIII and factor IX preparations and screening of donors on the basis of history 6 However an extremely small risk of acquiring AIDS through transfusion of seronegative blood persists because a recently infected individual may be antibodynegative 7 Currently this risk is estimated to be 1 in more than 2 million units of blood transfused As alluded to earlier mother to infant transmission is the major cause of pediatric AIDS 1 Infected mothers can transmit the infection to their offspring by three routes 1 in utero by transplacental spread 2 during delivery through an infected birth canal and 3 after birth by ingestion of breast milk 3 Of these transmission during birth intrapartum and in the immediate period thereafter peripartum is considered to be the most common mode in the United States 4 The reported transmission rates vary from 7 to 49 in different parts of the world 5 Higher risk of transmission is associated with high maternal viral load and low CD4 T cell counts as well as chorioamnionitis 6 Fortunately antiretroviral therapy ART given to infected pregnant women in the United States has virtually eliminated mother to child transmission but it remains a major source of infection in areas where these treatments are not readily available 7 Much concern has arisen in the lay public and among health care workers about the spread of HIV infection outside the high risk groups 8 Extensive studies indicate that HIV infection cannot be transmitted by casual personal contact in the household workplace or school 9 Spread by insect bites is virtually impossible Regarding transmission of HIV infection to health care workers an extremely small but definite risk is present 10 Seroconversion has been documented after accidental needle stick injury or exposure of nonintact skin to infected blood in laboratory accidents 11 After needlestick accidents the risk of seroconversion is believed to be about 0 3 and antiretroviral therapy