Some of the most familiar minor illnesses, such as coughs, sore throats, and attacks of diarrhoea and vomiting, are often caused by viral infections. However, viruses are responsible not only for minor infections but also for potentially fatal diseases, such as rabies and HIV infection and AIDS.
The first articles discuss frequently occurring viral infections such as the common cold and influenza, the related infections chickenpox and herpes zoster, and herpes simplex infections. The next articles describe the once-common viral infections of childhood: measles, mumps, and rubella. Until the second half of the 20th century, almost all children experienced these illnesses. Routine immunization has now made such diseases rare in developed countries. However, some of these viruses can still affect adults who have not been immunized, producing symptoms that are often severe. Articles follow on a number of viral diseases, including yellow fever and dengue fever, that may be encountered in the developing world. The section ends with an article on HIV infection and AIDS.
Viral infections that affect only one body system are covered elsewhere in the guide. For example, acute hepatitis and chronic hepatitis are covered in liver and gallbladder disorders, and genital herpes and genital warts are covered in sexually transmitted infections.
For more information on the structure and function of viruses.
A large group of diseases is caused by bacteria entering the body and multiplying too fast to be destroyed by the immune system. Some types of bacteria also release powerful poisons, known as toxins, that rapidly damage tissues. In the past, bacterial diseases were a major cause of death; today, most serious infections can be treated effectively with antibiotics.
Each of the bacterial infections covered in this section affects many areas of the body simultaneously. The most serious of these infections is septicaemia, described in the first article, which can be due to almost any bacterium. The diseases in the following articles are caused only by particular bacteria. They include both relatively recently identified infections, such as toxic shock syndrome, and those that have long been recognized, such as diphtheria and plague. The final articles describe illnesses caused by the rickettsiae bacteria, some of which may be transmitted to humans through insect bites.
Bacterial skin infections, such as boils, are discussed in skin infections and infestations.
For information on the structure and function of bacteria.
Protozoa and fungi are simple organisms, capable of living in many different habitats. Some protozoa and fungi are parasites of humans. They acquire all their food from our bodies and often cause disease. One protozoal infection, malaria, affects hundreds of millions of people worldwide each year and is often fatal.
Protozoal infections are discussed first, starting with malaria, the most important health hazard for visitors to the tropics. Further articles deal with other protozoal infections, including diseases that are common causes of diarrhoea, such as amoebiasis and cryptosporidiosis. The protozoal infection trichomoniasis, which is transmitted sexually, is covered with other such sexually transmitted infections (see Trichomoniasis). Fungal infections are discussed next. Like most protozoal infections, fungal infections may be serious in people with reduced immunity, such as those who have AIDS. The types of fungi described in this section can spread around the body from the initial site of infection, sometimes causing long-term illness. Common fungal infections that affect particular areas of the body, such as the skin and vagina, are covered in the sections on specific body systems.
For more information on the structure of protozoa and fungi.
Most animals, including humans, can be infested with parasitic worms that derive all their nutrients from their hosts. Most of these worms live in the intestines for at least part of their life cycle. In many cases, worm infestations are long-term diseases that produce few symptoms in their early stages.
Various types of the family of worms known as roundworms can infest humans. Most affect only people in the developing world. However, some roundworms, such as threadworm, commonly affect people in developed countries. The first five articles in this section describe several types of roundworm infestation: threadworm infestation, toxocariasis, ascariasis, hookworm infestation, and tropical worm infestations, such as filariasis.
Infestations in humans that are caused by other types of worms known as flatworms, which include tapeworms and flukes, are covered in the last three articles on schistosomiasis, tapeworm infestation, and hydatid disease.
It is possible to acquire a tropical worm infestation abroad but not to experience symptoms until some months later. If you become ill after visiting the tropics, you should visit a doctor familiar with tropical diseases.
For more information on the structure of worms
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.