The Histories of HIVs forms part of a series called ‘Perspectives on Global Health’, whose editor is James L. A. Webb, Jr. In this contribution, William Schneider has brought together nine academics over a wide spectrum of disciplines to share their knowledge on the origins of HIV. Most people with an above-average knowledge of HIV and AIDS are aware of the two types of HIVs: HIV-1 and HIV-2. These two viruses are genetically distinct, the HIV-1 virus having originated from the simian immunodeficiency virus (SIV) of a chimpanzee species found in Central Africa. HIV-2 originated in a similar SIV of sooty mangabey monkeys, which are found 2,000 miles away in West Africa. What is not common knowledge is that HIV-1 has four different strains (groups M, N, O and P), while HIV-2 has nine different strains (groups A to I). The first chapter gives more information about these different strains and their significance. Furthermore, the book investigates how the crossover from animals to humans took place. This is especially important for future research on zoonotic viruses – as the emergence of SARS-CoV-2, which led to the Covid-19 pandemic, clearly emphasized. As François Simon from the Faculty of Medicine, University of Paris, remarks in the Preface, ‘The history of HIVs is key to understanding the future of the Covid-19 epidemic’ (p. xii).

To combat or prevent a viral epidemic, it is important to understand how a virus is transmitted between humans. Once again, the recent measures implemented to prevent or at least slow down the transmission of SARS-CoV-2 illustrate this principle. In the years immediately following the identification of the HI virus in 1983, transmission often took place through the transfusion of contaminated blood. Blood transfusion carries the highest probability (92.5 per cent) of transmission, although the risk today is extremely small as blood services worldwide now ensure that they do not supply blood which carries a risk of transmitting HIV. Mother-to-child in utero also carries a high transmission risk, second only to transmission through blood transfusion. What I found surprising while reading this book was to learn of the relatively small transmission risk involved with heterosexual intercourse – for females, it is only around 0.08 per cent, and for males, it is 0.04 per cent. However, sexual intercourse still remains the most frequent means of transmission of HIV among humans. According to Ernest Drucker, one of the book’s authors, this means of transmitting the virus was exacerbated by the migration of people from rural villages to newly developing towns and cities as well as an increase in extramarital sexual relations, especially prostitution.

If heterosexual intercourse carries such a relatively low risk of HIV transmission yet remains the most frequent means of transmission, this raises questions about the disease’s spread. Drucker explains that, unlike other diseases that have a rapid growth in viral load (such as influenza, smallpox and Ebola), with symptoms that manifest themselves shortly after initial infection, HIV infections show very few distinctive signs in the early stages of infection, apart from possible flu-like symptoms, which are common in Africa. As soon as the HI virus enters the body, the immune system begins a vigorous battle to eliminate the invading organisms by reducing the viral load for an extended period, sometimes as long as ten years. During this time, the virus is transmissible between humans, albeit at a relatively low rate due to the reduced viral load, and many people may be unaware that they are even infected.

The book also tackles various theories on how the virus that causes AIDS crossed over from simian hosts to humans. Although there is no final conclusion, a number of possibilities are discussed, including the ‘cut hunter’ theory (which suggests that a hunter may have accidentally been infected while injuring themselves handling a chimpanzee carcass) and theories that point to the close contact between humans and primates in forests, where both forage for wild fruit. Fruit that has been bitten into by primates may still be collected and used by humans, according to contributors Tamara Giles-Vernick and Stephanie Rupp, thus opening up the possibility for viral transference. As humans expanded their gardens and as animals, including primates, foraged those gardens in search of maize, bananas and other fruit trees, new zones for infectious overlap were created.

How the virus then initially started spreading among humans is the theme of the next three chapters, focusing on Kinshasa, Cameroon, Côte d’Ivoire and Guinea-Bissau. Schneider, the editor, concludes the book by summarizing the main hypotheses of the various contributors and evaluating their differing viewpoints.

Anyone with an interest in the emergence and spread of HIV will benefit from reading this book. More importantly, this knowledge will greatly benefit epidemiologists as we await a possible new zoonic virus.