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Whenever a seemingly simple but intractable problem arises, a commonly heard refrain is ‘if we can put a man on the moon, why can’t we do X, Y, or Z’. As we have seen, parasitic diseases cause suffering to us and to our domestic animals, and the economic costs are enormous. Furthermore, many diseases could be controlled by simple measures such as providing safe drinking water and appropriate waste disposal facilities. So, one might ask, why do parasitic diseases continue to afflict so many people and impact so heavily on agriculture?

As with so many apparently simple questions, the reason parasitic diseases remain a problem does not have a single simple answer and is also tied up with the most exasperating factor of all – human behaviour (Table 1.3). To begin with, human parasitic diseases are predominantly (although not entirely) a problem of poor people who live in insanitary conditions and who do not have a healthy diet. The diseases are therefore most prevalent in developing countries where neither the government nor individual people have money to spare. For example, in 2016 the total healthcare expenditure in Zimbabwe as a percentage of the gross domestic product (GDP) was similar to that of the United Kingdom (9.41% cf 9.76%) and considerably more than that of oil‐rich Saudi Arabia (5.74%) (https://data.worldbank.org/indicator/SH.XPD.CHEX.GD.ZS). However, in terms of total health expenditure per capita, the United Kingdom spent US$4192, Saudi Arabia US$1147, and Zimbabwe US $94 (https://knoema.com/atlas/Zimbabwe/Health‐expenditure‐per‐capita). Needless to say, US $94 does not buy many medicines.

Table 1.3 Summary of factors contributing to the problems of parasitic diseases.

Poverty Lack of sanitation Complacency Poor nutrition Lack of health infrastructure Lack of government interest Corruption Urbanization Social conflict/wars Movement of non‐immune people to regions where they become infected from the resident population. Movement of infected people to regions where they infect non‐immune resident population Man‐made environmental damage Natural disasters Lack of effective drugs/ parasite resistance Increasing resistance of vectors/ intermediate hosts

We humans are extremely adaptable creatures. Consequently, we can survive harsh environments, oppressive regimes, and cruel exploitation. Unfortunately, this adaptability can degenerate into acceptance and complacency on the parts of both individuals and governments. Because parasitic diseases are so prevalent in developing countries, there is a tendency not to prioritise them: fevers and diarrhoea become an accepted part of everyday life. Furthermore, parasitic diseases tend to cause chronic disease and although the patient may ultimately die, the condition does not capture the attention of the local or world media. For example, Ebola virus is well known in the developed world because of its appalling pathology and images of patients being treated by nurses and doctors dressed in spacesuit‐like protective clothing. However, although Ebola virus causes about 70% mortality, the numbers of people who have died of the infection are relatively few. By comparison, Human African Trypanosomiasis (HAT, often referred to as ‘sleeping sickness’) causes almost 100% mortality if untreated and kills many more people than Ebola (Table 1.2), but it seldom receives a mention in the media. The reason is simple, HAT kills slowly by comparison. Furthermore, the transmission of HAT depends upon tsetse flies, and these have demanding environmental requirements that limit their distribution. Consequently, HAT is only a threat to people living in certain parts of Africa. By contrast, Ebola spreads through close human contact and therefore the virus could conceivably spread anywhere in the world. Consequently, people in distant countries feel threatened even though their risk is incredibly small. The fact that Ebola virus has been touted as a possible biological warfare agent also helps to engender interest in the disease and funds to study and control it.

In addition to being poor, the countries in which parasitic diseases are most problematic are often unstable and suffer high levels of corruption. Consequently, those in control often devote much of their revenue into the trappings of power and military spending: many developing countries spend less than 4% of their GDP on healthcare. This means that even less of not very much is available for the treatment and control of parasitic diseases. The instability of the regimes and conflicts, which can last for decades makes it difficult to provide health services and co‐ordinate control strategies. They also lead to the destruction of basic infrastructure and the decline in agricultural and commercial activity – and this contributes to poverty and malnutrition. At its worst, conflicts lead to large numbers of refugees who are frequently housed in squalid campsites, which lack proper sanitation. These displaced people are often in poor health and malnourished, they take their parasites with them wherever they go, and they are highly vulnerable to the local strains of parasites at wherever they arrive. For example, the civil wars in the Central Asian states such as Tajikistan, which occurred after the breakup of the Soviet Union in the early 1990s, displaced people to neighbouring countries including Afghanistan. The most common type of malaria in Tajikistan at that time was caused by Plasmodium vivax, whereas in Afghanistan, the more virulent Plasmodium falciparum was found, and drug‐resistant strains were circulating. Some of the refugees who returned home in the late 1990s were infected with drug‐resistant P. falciparum and since there was a suitable mosquito vector, this form of malaria was subsequently transmitted among people who had never left Tajikistan (Pitt et al. 1998). Similarly, at the time of writing, the wars in Syria and Yemen had resulted in an almost complete collapse of their health infrastructure. In both Syria and Yemen, leishmaniasis was becoming a serious problem, and the disease was being transmitted to refugee camps in surrounding countries (Al‐Salem et al. 2016; Du et al. 2016). Syria also saw a rise of almost 100,000 cases of malaria between 2015 and 2016 (https://www.globalcitizen.org/en/content/malaria‐yemen‐crisis‐increasing‐cases/) whilst in the Yemen, control programmes that aimed to eliminate onchocerciasis and lymphatic filariasis by 2015 foundered with no prospect of them resuming (Abdul‐Ghani 2016).

Natural disasters, such as cyclones and earthquakes, can lead to similar destruction of infrastructure and refugee problems to those of war. Widespread flooding also provides extensive breeding conditions for mosquitoes and thereby increases the spread of mosquito‐borne diseases such as malaria. The destruction of sewage systems and facilities for waste disposal, in conjunction with a warm wet environment, also facilitates the spread of faecal‐oral transmitted protozoa and helminths. It is therefore not surprising that widespread flooding in tropical countries usually results in an increase in malaria and water‐borne diseases (Boyce et al. 2016; Okaka and Odhiambo 2018).

The damage we cause to the environment can encourage the spread of disease by making conditions more suitable for vectors and intermediate hosts and/or the survival of parasite eggs and cysts. For example, clearance of the rainforests in the Amazon produces open sunlit pools that are ideal breeding grounds for the mosquito vector of malaria Anopheles darlingi (Harris et al. 2006). Also, as people move into these clearings to live or work, they come into contact with zoonotic infectious agents that may not be perfectly adapted to living in us but can still cause disease.

The way we live and organise our societies is a major contributor to the spread of parasitic diseases. Throughout the world, there is an increase in urbanization. This means that more people are living close together and the potential for disease transmission between them is therefore high (McMichael 2000). Vector species that can live in an urban environment, such as Anopheles stephensi and certain other mosquitoes, therefore pose a particular risk (Takken and Lindsay 2019).

If a high population density combines with inadequate sanitation, then widespread transmission of contaminative diseases is inevitable. In some slums, over 50 households may share a single toilet. Furthermore, this toilet may be 50 m or more from the dwellings. Consequently, urinating and defecating on the bare ground by both children and adults are common in some of these communities. In a study of slum dwellers in Gujarat (western India), 71% of the participants were infected with parasitic protozoa and 26% with helminth infections (Shobha et al. 2013). Not surprisingly, many claimed to suffer from diarrhoea. Similarly, a study of slum children (1–5 years old) in Karachi (Pakistan) found that the prevalence rate of intestinal parasites was 53 and 10% of the children harboured two or more parasite species (Mehraj et al. 2008). Many of these children suffered from stunted growth.

Sometimes, parasites and their vectors spread by less obvious means. For example, the increased use of cars and motorised transport has resulted in large numbers of used tyres entering the ecosystem. Used tyres retain water after it has rained, and they make excellent breeding grounds for some mosquito species. There is a huge international market in used tyres that are loaded onto lorries and ships and moved within and between countries. In the process, mosquitoes are also moved around the world and notorious vectors of disease such as the Asian tiger mosquito Aedes albopictus are now established in countries such as Spain where they were formerly absent. Aedes albopictus does not transmit parasitic diseases but is an important vector of viruses such as Dengue virus, yellow fever virus, and Zika virus. The adults are not capable of dispersing far by flight, but it has colonized many countries through the transport of its larvae in used tyres. The adult mosquitoes also disperse by unintentionally hitching a ride inside a car or other vehicle (Eritja et al. 2017). It is likely that many other mosquitoes and other vectors disperse in similar fashions. For example, there are several reports of ‘airport malaria’ in which a person contracts the disease from a mosquito that has been carried from one country to another within a plane (Isaäcson and Frean 2001).

Before the COVID‐19 pandemic that began in 2019, people were increasingly mobile and cheap air travel meant that millions of people rapidly moved between countries for leisure and business. In addition, large numbers of people moved long distances as economic migrants and political refugees. The COVID‐19 pandemic brought much of this movement to a sudden halt, and at the time of writing, it was uncertain when and to what extent mass movements will return. Anyone who moves to a new environment becomes exposed to diseases to which they have no previous experience, and hence immunity. They are therefore vulnerable to infection. Similarly, those who are already infected (but may not be aware of the fact) carry their diseases with them and could potentially transmit their infections to a non‐immune population on arrival. Obviously, when many people are moving there are many opportunities for disease transmission. For domestic animals, it is possible to instigate legislation that governs their movement. For example, a passport scheme can ensure that they have received appropriate vaccinations and/or drugs to remove infections. Similarly, a period of quarantine upon arrival at their destination can be imposed. Except in very authoritarian regimes, this is seldom feasible as a long‐term solution for human populations. Although some countries closed their borders and/or imposed strict quarantines on people during the COVID‐19 pandemic, this approach cannot be sustained for any length of time because of the economic consequences. Some countries insist that all persons entering their borders have documentation proving they have received certain vaccinations, such as for yellow fever. However, there are few anti‐parasite vaccines and even where effective prophylactic medicines are available to treat parasites, such as anti‐malarial drugs, it is notoriously difficult to persuade people to take them as prescribed.

Another of the major reasons why parasites remain a problem is the lack of suitable drugs and vaccines to treat them. The development of drugs for use in human medicine takes many years and is extremely expensive. Consequently, the drug companies need to be sure that they will obtain a good rate of return for their investments. See Chapter 14 for more information on the treatment of parasitic diseases. Unfortunately, those who suffer most severely from parasitic diseases are usually poor and cannot afford expensive drugs. Similarly, the development of anti‐parasite vaccines is hampered by a combination of cost and the difficulty of generating protective immunity against parasitic infections. These issues are dealt with in detail in Chapter 15.

The control of parasites by targeting their vectors/intermediate hosts is also becoming more problematic. For many years, this approach proved highly effective, and in the 1950s, it was even believed possible that malaria might be eradicated by killing the anopheline mosquito vectors. However, some vectors are exhibiting increasing resistance against a wide range of insecticides and new chemicals are not being developed to replace those in current use. Furthermore, there are mounting concerns for the environmental damage that can result from inappropriate use of insecticides and fears over risks they pose to our health.