This report is a summary of Sections 3.1 to 3.10 of Chapter 3 – “Infectious Diseases and Treatments” – of “Biology of Disease” (Ahmed, Dawson, Smith & Wood, 2007).
Section 3.1: Introduction. A small number of pathogenic microorganisms can cause systemic disease – affecting the entire body. This chapter looks at selected examples of major body systems infections, including microorganisms that can cause generalized infections.
Section 3.2: Infections of the Skin. As an important part of the body’s immune defense system, the skin is normally populated by various microorganisms, which effectively prevent access by invading microorganisms such as Papillomaviruses which causes warts, and various herpes virus types. These include genital herpes and others that cause chickenpox and shingles. When a bacterial skin infection does occur, it is usually because there is a disturbance of the balance between these organisms and the skin. A typical example is the incidence of acne due to hormonal imbalances during puberty. Also, in children, skin breaks can bring on impetigo contagiosa, which is an extremely contagious skin infection.
Section 3.3: Infections of the Eyes, Ears and Central Nervous System. Infections of the eyelids generally occur in the margins of the lids, or in the lid glands or follicles, causing styes, and are usually linked with Staphylococcus aureus. Trachoma is globally the most significant eye infection, affecting circa 500 million people, blinding some. The inner eye can be seriously infected by Toxoplasma Gondii – a protozoan that can cause blindness. Parasitic worms such as the tapeworm are another source of eye infections, and river blindness – prevalent in Africa and Central America and caused by larvae carried by the Simulium fly – affects more than 300,000 people globally, and often causes blindness. Outer ear infections may have some antibiotic resistance. Middle ear infections can be caused by viruses or bacteria from the upper respiratory tract. The resulting otitis media is treated by oral antibiotics to prevent possible perforation of the eardrum and hearing defects.
Central Nervous System (CNS) infections can affect multiple areas simultaneously, including the spinal cord and the brain, causing diseases such as meningitis, encephalitis and encephalomyelitis. These infections may be caused by viruses or bacteria.
Section 3.4: Infections of the Respiratory System. Although the inhalation of microorganisms continually exposes the respiratory system to potential infections, the defense systems include numerous immune defenses and the nasal passages, which filter out larger particles. Nonetheless, respiratory tract infections are common, through viruses transmitted directly in breathed air, or indirectly by contact with contaminated objects. Possible diseases include Sinusitis, Rhinitis, Laryyngitis, Epiglottitis, Tracheitis and Bronchitis. The nose and throat are primary infection sites, but secondary bacterial infections can produce complications such as diphtheria, whooping cough, pneumonia and tuberculosis.
Section 3.5: Infections of the Gastrointestinal Tract (GIT). Whilst all regions of the GIT are potential sites of infection, saliva and stomach acid remove many pathogens, although more enter by breathing and eating. Oral cavity infections such as actinomycosis (inflammation caused by fungal infections that can be caused following injury) are different to those affecting the stomach or the intestines. Some bacteria are saliva-resistant, causing biofilms or microcolonies to be created; dental plaque is one example. Periodontal (gum) disease such as gingivitis is another. If untreated, that can lead to periodontitis, or pyorrhea.
Stomach and intestinal infections are caused by “viruses, bacteria, protozoa and worms,” which can all come from contaminated food or water or by “fecal-oral contact.” The resultant diarrhea can be fatal in a significant percentage of cases for young children.
Section 3.6: Infections of the Urogenital System. Specific pathogens cause infections of the urinary and genital systems respectively. Urinary Tract Infections (UTIs) such as Cystitis are more common in women than in men, due to their shorter urethra, and hormonal changes linked to the menstrual cycle. Other UTIs can be triggered after hospitalization, or surgery and can resist antibiotics. And Thrush in the bladder can occur following antibiotic treatments. Sexually Transmitted Diseases (STDs) can cause infections of the urinary and genital systems. This Section of the chapter devotes an entire sub-section to AIDS, and includes discussion of other STDs such as gonorrhea and syphilis.
Section 3.7: Sepsis and Systemic Infections. Resulting from just a small number of pathogens entering the body via the lymphatic or circulatory systems, generalized or systemic infections can affect several body organs such as the “brain, bone marrow, kidneys, liver, lungs and spleen.” Some acute generalized infections can develop quickly, passing through four discrete stages. Stage 1 is characterized by increased heart rate, body temperature and respiration. Stage 2 is called sepsis, when the infecting organism can be found in the blood. If not cured at this point – either by the body’s immune system or by antibiotics – the condition progresses to Stage 3, serious sepsis, otherwise called multi-organ dysfunction syndrome (MODS). The blood pressure falls and there is evidence of lactic acidosis, hypoxia and oliguria. That can be followed by Stage 4, septic shock, which frequently leads to death from irreversible organ failure. Other general infections developing from untreated localized infections have an incubation period of up to three weeks. Typhoid fever is one example.
Section 3.8: Investigating Infectious Diseases. Two criteria needed to diagnose an infectious disease (by examination and testing) are the expected signs and symptoms, and the recovered pathogen or evidence of its existence at the infection site.
Section 3.9: Preventing Infectious Diseases. Vaccines are the main weapons preventing infectious diseases. They cause the body’s immune system to build antibodies to fight the targeted infection(s).
Section 3.10: Controlling the Spread of Pathogens. Control is complex, including microbiological investigations, strict hygiene procedures, prescribing antibiotic or antiviral drugs, and even vaccination or drugs for all patient contacts.