8. Looking ahead

‘Looking ahead’ shows how our understanding of disease ecology and evolution has revolutionized disease management. By developing transmission control strategies to close the encounter filter and vaccines and treatments to close the compatibility filter, we have reduced the misery caused by infectious disease. But what is the outlook for the future control of infectious diseases? We cannot eradicate infectious disease. Living things have parasitized one another since the beginning of life itself. New zoonotic diseases will continue to emerge, and existing diseases will continually evolve to escape our methods of control. Despite this stark reality, we can minimize the impact of disease even if we can never fully conquer it.

6. Malaria

Malaria is transmitted to humans through various species of Anopheles mosquitoes. In this century malaria rarely reaches out of the tropics, being limited by the ecological niche of its mosquito vectors. The most widespread strains of malaria are typically chronic and debilitating, rather than causing acute infection and death, but the cumulative impact of malaria on humanity is enormous. ‘Malaria’ considers the complexity of the disease; the history of human malaria; and the strategies employed against the disease, including the use of compatibility-blocking treatments such as quinine, chloroquine, and artemisinin, and encounter-blocking strategies such as residual indoor spraying and insecticide-treated bed nets. It concludes by considering the future for malaria control.

4. HIV

HIV is the human immunodeficiency virus that causes acquired immunodeficiency syndrome, or AIDS. Its transmission is by exchange of bodily fluids. HIV can only enter immune cells with the surface protein gp120. The virus can hide in these cells for many years before it is activated, although it can be transmitted throughout this period. Once activated, the virus begins to replicate, ultimately causing the immune system of the infected person to collapse making them vulnerable to opportunistic infections. ‘HIV’ describes how evolutionary biology has been used to clarify the origins of the epidemic. The rapid mutation rates and recombination that make HIV very hard to treat are also explained. Despite these challenges, a regimen of highly active anti-retroviral therapies (HAART), developed in the mid 1990s, is extraordinarily effective against HIV.

2. Transmission at different scales

The term ‘transmission’ defines infectious disease. Respiratory viruses such as influenza are airborne; diseases such as HIV and hepatitis are transmitted through direct, usually sexual, exchange of bodily fluids; water-borne diseases such as cholera can survive in the environment; and vector-borne pathogens have evolved to use other organisms, especially blood-sucking insects and mites, to travel from one host to another. ‘Transmission at different scales’ considers the filters for encounter and compatibility, mathematical modelling of epidemic dynamics, and the key factors of virulence, resistance, and tolerance.

1. Introduction

The ‘Introduction’ explains what infectious diseases are: diseases that are transmitted from one person to another. For most of human history, diseases could only be controlled at the population level, using quarantines to separate uninfected from infected people. The discovery of immunization, and later the invention of disease treatments like antibiotics, allowed individual-level infectious disease control. Individual-level control can filter up to the population level: if enough of the population are vaccinated, we can reduce transmission enough to stamp out a local epidemic, or even to wipe a disease out globally. But both humans and infectious disease agents are living organisms that undergo ecological and evolutionary change, making infectious disease a moving target.

7. Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a fungal pathogen that infects many different amphibian species, driving some of them to extinction. ‘Batrachochytrium dendrobatidis’ considers the physiology and natural history of this emerging pathogen; its discovery in the late 1990s in Australia and Central America; and the concepts and strategies used to try to determine its origins. Has it arrived relatively recently in the communities it was destroying, or did it lie dormant in those communities for millennia before suddenly beginning to cause harm? The debate between the novel pathogen hypothesis and the endemic pathogen hypothesis, versions of which apply to most emerging diseases of wildlife, is ongoing.

3. Influenza

Influenza, or flu, has caused more deaths than any single disease outbreak since the 14th-century bubonic plague. Twenty to fifty million people worldwide died from the 1918 Spanish Flu, which was an H1N1 strain similar to the 2009 pandemic. ‘Influenza’ considers the attempts to control transmission of flu by reducing encounters, by reducing compatibility (through vaccination), or ideally by a combination of both. The unique evolutionary potential of flu means that both vaccinated people and unvaccinated people who contract flu naturally tend to lose their immunity after a few years. General principles of evolutionary biology, as well as intriguing particulars of flu evolution, are introduced. The politics of research and risk assessment are also discussed.

5. Cholera

Cholera is a water-borne, diarrhoeal disease caused by the bacterium Vibrio cholerae. For illness to occur, humans must consume vast amounts of the bacteria and cholera must possess a cluster of genes involved in creating the toxin co-regulated pilus. The gene for cholera enterotoxin must also be expressed. Treatment for cholera involves giving victims oral rehydration and antibiotics as a secondary defence. ‘Cholera’ explains that cholera gave rise to the foundation of epidemiology and Koch’s postulates, which state the requirements for identifying a particular organism as the causative agent of a disease. It also describes the ways in which cholera evolves antibiotic resistance, the interactions between Vibrio cholerae and the viruses that prey upon it, and possible reasons for the seasonality of epidemics.