The study of all aspects of micro-organisms (microbes) – that is, organisms which are generally too small individually to be visible other than by microscopy. The term is applicable to VIRUSES, BACTERIA, and microscopic forms of FUNGI, algae, and PROTOZOA.

Among the smallest and simplest micro-organisms are viruses which can be seen only by electron microscopy. They consist of a core of deoxyribonucleic or ribonucleic acid (DNA or RNA) within a protective protein coat, or capsid. Larger viruses (pox-, herpes-, myxo-viruses) may also have an outer envelope. Their minimal structure dictates that viruses are all obligate parasites, that is they rely on living cells to provide essential components for them to be able to replicate. Apart from animal and plant cells, viruses may infect and replicate in bacteria (bacteriophages) or fungi (mycophages), which they damage in the process.

Bacteria are larger and more complex. They have a subcellular organisation which generally includes DNA and RNA, a cell membrane, organelles such as a RIBOSOME, and a complex and chemically variable cell envelope – but no nucleus. Bacteria may also possess additional surface structures, such as capsules and organs of locomotion (flagella) and attachment (fimbriae and stalks). Individual bacterial cells may be spheres (cocci); straight (bacilli), curved (vibrio), or flexuous rods (spirilla); or oval cells (coccobacilli). On examination by light microscopy, bacteria may be visible in characteristic configurations (as pairs of cocci [diplococci], or chains [streptococci], or clusters). Bacteria grow by increasing in cell size and dividing by fission, a process which in ideal conditions some bacteria may achieve about once every 20 minutes.

Eukaryotic (nucleus-containing) micro-organisms comprise fungi, algae, and protozoa. Fungi grow either as discrete cells (yeasts), multiplying by budding, fission, or conjugation, or as thin filaments (hyphae) which bear spores, although some may show both morphological forms during their life-cycle. Algae and protozoa generally grow as individual cells or colonies of individuals and multiply by fission.

Many bacteria and most fungi are saprophytes (see SAPROPHYTE), being major contributors to the natural cycling of carbon in the environment and to biodeterioration; others are of ecological and economic importance because of the diseases they cause in agricultural or horticultural crops or because of their beneficial relationships with higher organisms. Additionally, they may be of industrial or biotechnological importance. Fungal diseases of humans tend to be most important in tropical environments and in immuno-compromised subjects.

Pathogenic (that is, disease-causing) micro-organisms have special characteristics, or virulence factors, that enable them to colonise their hosts and overcome or evade physical, biochemical, and immunological host defences. For example, the presence of capsules, as in the bacteria that cause anthrax (Bacillus anthracis), one form of pneumonia (Streptococcus pneumoniae), scarlet fever (S. pyogenes), and bacterial meningitis (Neisseria meningitidis, Haemophilus influenzae) is directly related to their ability to cause disease because of their antiphagocytic properties. Fimbriae are related to virulence, enabling tissue attachment – for example, in gonorrhoea (N. gonorrhoeae) and cholera (Vibrio cholerae). Many bacteria excrete extracellular virulence factors; these include enzymes and other agents that impair the host's physiological and immunological functions. Some bacteria produce powerful toxins (excreted exotoxins or endogenous endotoxin), which may cause local tissue destruction and allow colonisation by the pathogen or whose specific action may explain the disease mechanism. In Staphylococcus aureus, exfoliative toxin produces the staphylococcal scalded-skin syndrome, TSS toxin–1 toxic-shock syndrome, and enterotoxin food poisoning. The pertussis exotoxin of Bordetella pertussis, the cause of whooping cough, blocks immunological defences and mediates attachment to tracheal cells, and the exotoxin produced by Corynebacterium diphtheriae causes local damage causing a pronounced exudate in the trachea.

Viruses cause disease by cellular destruction arising from their intracellular parasitic existence. Attachment to particular cells is often mediated by specific viral surface proteins; mechanisms for evading immunological defences include latency, change in viral antigenic structure, or incapacitation of the immune system – for example, destruction of CD 4 lymphocytes by the human immunodeficiency virus.