This chapter evaluates the defences that make up the human immune system, which are crucial to our ability to fight off infectious diseases. The innate immune system has a variety of defences to prevent pathogens from infiltrating the body. Macrophages such as mast cells, dendritic cells, basophils, and eosinophils aid in recognizing pathogens and chemically signalling to other aspects of the immune system, such as phagocytes, T-cells, and B-cells. The T-cell and B-cell pathways are known as the adaptive immune response, as they are capable of adapting to new pathogens. They also 'remember' antigens, through the use of T and B memory cells, allowing the body to quickly respond to any pathogens that it has encountered before. The chapter then looks at autoimmune disorders, which are caused when the immune system fails to recognize other cells within the body and attacks them in the same way that it attacks pathogens.
Defending against the invaders
Fighting against infection: a chemical approach
This chapter highlights the advances in medicine and microbiology over the past century which have led to huge advances in the way we treat infectious diseases—and how we can prevent them from occurring in the first place. These include antibiotics, antivirals, antifungal medications, and antiparasitic medications. Antibiotics are either bacteriostatic (halting division and growth of bacteria without killing them) or bacteriocidal (acting to destroy bacteria directly). They have a number of different common mechanisms of action, with many antibiotics working against multiple different types of bacteria. Meanwhile, antiviral medications must work to inhibit all reproduction of the virus otherwise resistant mutants will quickly appear. Many viral infections cannot be treated with antiviral agents, as each drug must be very specific to the individual virus (or small group of viruses).
Fighting against infection: vaccination and public health
This chapter focuses on vaccination, which may be one of the most successful aspects of public health programmes worldwide. Vaccinations work by introducing the body to a dead or weakened version of a pathogen. This allows the T and B memory cells of the adaptive immune system to store the 'memory' of these specific antigens, allowing the immune system to quickly respond if the host meets the same pathogen again in the future. Herd immunity helps to reduce the spread of infectious disease through a community. The more individuals are immunized, the harder it becomes for a disease to spread to people who are still vulnerable to it. Some basic public health measures—such as street cleaners and spittoons—have existed for a long time, but it is only relatively recently that governments and health organizations have started to track the spread and cause of infectious diseases.
Global outbreaks, a post-antibiotic landscape, and the evolutionary arms race
This chapter reflects on the future of infectious diseases. Improving global transport links, as well as the demand for food from all over the world, makes it easier than ever before for diseases to spread over great distances. Worldwide organizations such as the World Health Organization (WHO) aim to coordinate the efforts of local, national, and international public health organizations to track, treat, and prevent infectious disease. However, these efforts can sometimes be hindered by cultural or political boundaries, and sometimes disease can even be inadvertently spread by local customs or superstitions. Infectious diseases have evolved alongside humanity, and are constantly involved in an 'evolutionary arms race'. They develop ways to subvert our defences, or even manipulate our behaviour for their own good. New antibiotics are both difficult and expensive to discover, and many infections are now developing resistance to multiple different antibiotics.
Human Infectious Disease and Public Health starts off by looking at the historical development of our understanding of human infectious diseases, which it describes as ‘the silent enemy’. It then moves on to consider a number of important pathogens such as bacteria, fungi, viruses, parasites, prions, and cultures. This overview of pathogens is followed by a chapter which looks at the different defence mechanisms with which the human immune system fights off disease. The next two chapters cover how developments in the medical field have helped with fighting infections, firstly in terms of medication and, secondly, in terms of vaccination. The final chapter looks at global outbreaks and asks what a post-antibiotic landscape would look like.
A pantheon of pathogens 1: Bacteria, fungi, and viruses
This chapter discusses three types of pathogens: bacteria, fungi, and viruses. Bacteria may be grouped taxonomically, or according to shape, oxygen dependence, or staining. Many bacteria do not cause infectious diseases in humans; those which do cause disease may lead to mild, localized infections or severe, life-threatening ones. Meanwhile, fungi are grouped taxonomically. They rarely cause infections in humans with functioning immune systems, but can take advantage of individuals with lowered immunity to cause severe symptoms of disease. Finally, viruses have a variety of methods for infiltrating their host's cells, but cannot reproduce without using the organelles within these cells, making them obligate parasites. They will often damage or destroy the cell once they have replicated. Viruses tend to be classified either using classical taxonomic methods or by the Baltimore classification.
A pantheon of pathogens 2: Parasites, prions, and cultures
This chapter examines parasites, prions, and cultures. Parasites take many different forms—some are multicellular and very large in size, others microscopic and difficult to detect. When parasites infect us, they always have some effect on us—usually, but not always, causing an infectious disease. Meanwhile, prions are misshapen proteins which can induce similar misfolding in other proteins throughout the body. They cause progressive, degenerative, and irreversible neurological symptoms, and are very hard to detect and diagnose. The chapter then looks at different laboratory techniques for growing or identifying different types of pathogen, which are vital for diagnosis of disease and the selection of the appropriate treatment. It also considers the concept of biosecurity, the term used for a set of precautions that aim to prevent the introduction and spread of harmful organisms.
The silent enemy
This chapter traces the evolution of infectious diseases. Infectious diseases have evolved alongside humanity, taking advantage of changes in human society to help them to spread from person to person. Many early theories of disease involved religious or magical constructs. Moreover, in many early models, diseases were described in terms of an imbalance of the four humours. Some historical attempts to treat disease may have had some degree of success, as some of the herbs used in traditional medicines have antimicrobial properties, and people began to understand the principles of quarantine. An understanding of the nature of infectious disease has developed over time, but linking the discovery of microorganisms to the theory of infectious disease (known as the germ theory of disease) was a slow process, taking many different scientists many hundreds of years to develop.