Pathogens and Disease

Towards the end of 2011 some farmers in Northern Europe reported reduced milk yields and some animals not making the expected weight at slaughter. The impact had been notable but not dramatic and the reason was unknown. With the coming of spring 2012, the farming community was looking forward to lambing and calving season. But on a small number of farms, a potentially devastating situation began to unfold. Healthy, pregnant animals unexpectedly had abortions or had stillborn offspring with fused limbs, malformed bodies, and misshapen heads. The culprit: Schmallenberg virus (SBV).

Chronic Obstructive Pulmonary Disease (COPD) is a diverse respiratory disorder whose main characteristics include the persistent obstruction of the small airways of the lungs, the destruction of alveolar tissue and chronic bronchitis. It is estimated that about 328 million people suffer from COPD worldwide, but this number will soon rise significantly. The disease is projected to rank third in the most fatal disease table by 2020 and predicted to account for 7% of total deaths by 2030.

Aspergillus fumigatus is an opportunistic fungal pathogen. The spores of this fungus, called conidia, are found widely in the environment and it is thought that we inhale several conidia daily. When we inhale conidia, resident immune cells in our lungs phagocytose and destroy them − preventing them from causing an infection. Alveolar macrophages in the lung are the primary line of defence against conidia.

Candida albicans is a normal part of the human commensal flora, however it is also the most common fungal species that can cause human disease.

Chlamydia trachomatis (Ct) infection is the commonest bacterial sexually transmitted infection worldwide (Howie et al., 2011a, 2011b). In the under-25 age group in the UK 7–8% of men and women are infected. 70% of women and 50% of men who have Ct infection have no symptoms (Manavi, 2006); therefore if they do not get tested and treated, they can continue spreading the disease to their partners. These aspects of Ct infection can account for the distress that may occur when someone who had assumed that they were healthy, as they had no symptoms, discovers that they are infected when they happen to be tested for sexually transmitted diseases (STDs).

Any microorganism which is able to cause disease in a host organism is termed a pathogen. This article is confined to human microbial pathogens, although plant and animal pathogens are also widespread in nature. When a pathogenic microorganism (bacterium, virus or protozoal parasite) infects the human body, a battle ensues between the host’s innate & adaptive immune systems and the pathogen’s assorted virulence mechanisms and factors.

Human immunodeficiency virus (HIV), a member of the retrovirus family, is the causative agent of acquired immunodeficiency syndrome (AIDS). HIV invades various immune cells (e.g., CD4+ T cells and monocytes) resulting in a decline in CD4+ T cell numbers below the critical level, and loss of cell-mediated immunity − therefore, the body becomes progressively more susceptible to opportunistic infections and cancer.

Via complement-mediated lysis

When bacteria, such as Neisseria meningitidis, invade the body, they are attacked by immune proteins called complement proteins. Complement proteins assist in bacterial killing via three pathways, the classical complement pathway, the alternative complement pathway or the lectin pathway.

Human cytomegalovirus (HCMV) is a beta-herpesvirus that causes lifelong infection in humans. HCMV has a prevalence of 55-100% within the human population, depending on different socioeconomic and geographical factors. Primary HCMV infection is generally asymptomatic in healthy hosts but can cause severe and sometimes fatal disease in immunocompromised individuals and neonates. HCMV is the leading infectious cause of congenital abnormalities in the Wwestern world, affecting 1-2.5% of all live births. HCMV intrauterine infection and can cause significant morbidity, including low birth weight, hearing loss, visual impairment, microcephaly, hepatosplenomegaly, and varying degrees of mental retardation.

There are 1.5 - 5 million species of fungi which have the ability to grow almost anywhere including the ocean, soil, plants and animals. Some form spores which we inhale on a daily basis (e.g. Aspergillus species), and others live as human commensal organisms (e.g. Candida species). Despite the close encounters we have with fungi, how our immune system recognises and protects us from fungal pathogens is not as well understood as compared to bacteria or viruses.