What is an infection?
An infection is when a microbe or microorganism invades the body and replicates, resulting in illness and disease with symptoms including pain, inflammation, fever and sores, depending on the type of microorganism and where the infection is located. The infective organism is known as a pathogen and can enter the body by several routes, for example, through a wound, by inhalation, in body fluids, in contaminated food, or in a vector (another organism that transmits a pathogen).
An infection can be caused by:
- Bacteria, which are single cell organisms with a cell wall but no nucleus and all equipment needed to replicate their genetic material independently of the host cell.
- Viruses, which are much smaller than bacteria, are surrounded by a protein capsule and cannot replicate independently, instead they use the host cell enzymes to replicate their genetic material, which can be either DNA or RNA.
- Fungi, which are multi-cellular organisms that replicate independently and spread by growing hyphae, as in the case of mould-like fungi called dermatophytes such as tinea, or by budding as in the case of yeasts like candida.
Many bacteria live in the body without causing any harm, such as in the intestines where they help with digestion, or on the skin. However if bacteria invade the body, for example through a wound, by inhalation or in food, they can end up in a part of the body they are not meant to be and can become pathogenic by dividing and reproducing rapidly within the cells of the infected tissue. This is a bacterial infection which can cause illness with symptoms including pain, inflammation and sores, depending on the type of bacteria and where the infection is located. Different strains of some bacteria cause different types of infection and some bacteria produce toxins, which damage the surrounding tissues. A bacterial infection can be serious even life threatening if the bacteria get into the blood (septicaemia).
Antibiotics are medications used to treat bacterial infections. The mechanism of action varies between different classes of antibiotic and this determines how they work to prevent the spread of a bacterial infection. Antibiotics that target the bacterial cell wall, cell membrane or essential enzymes kill the bacteria and these antibiotics are called bacteriocidal, for example penicillins like amoxycillin, cephalosporins like cephalexin, sulphonamides like sulfamethoxazole and fluoroquinolones like ciprofloxacin. Others target protein synthesis, which prevents the bacteria from dividing but does not kill them and these antibiotics are called bacteriostatic, for example macrolides like erythromycin, Lincosamide like clindamycin, aminoglycosides like streptomycin and Tetracyclines like doxycycline. Some antibiotics have a broad spectrum of action against many different bacteria, whereas some have a narrow spectrum and are used specifically for certain families of bacteria.
Viral infections are due to invasion of the host cell DNA by a virus, which then uses the cell’s enzyme to replicate so that new virus particles can be produced and shed ready to infect another cell, killing the host cell in the process. The common cold is caused by infection of the upper respiratory tract usually by a rhinovirus, and infection of the same tissues with influenza virus causes influenza, which is often confused with a cold but the symptoms are much more severe and can be deadly. Human Papillomavirus (HPV) infects skin cells, causing warts and is also associated with cervical cancer. Herpes Simplex virus infects mucous membranes of the genitals (genital Herpes) and lips (cold sores). Varicella, a Herpes-like virus, causes chicken pox and also shingles (herpes zoster). Human immunodeficiency virus (HIV
) infects cells of the immune system and can cause acquired immunodeficiency syndrome (AIDS).
Viruses are not easy to treat, as unlike bacteria it is difficult to kill the virus without killing the body’s cells. However, there are antiviral drugs for treating viral infections, which work by several different mechanisms to prevent the growth and spread of the virus, but they do not kill the virus. An immunomodifier activates immune cells in outer layers of the skin to help fight invasion by a virus; other drugs kill infected skin cells. Some drugs target viral enzymes, for example, an influenza drug prevents new virus particles being released from infected cells. An enzyme that is needed to replicate newly synthesised viral DNA is another target. For the treatment of RNA viruses like HIV, drugs block the action of a specific viral enzyme that converts viral RNA to DNA so that it cannot become incorporated into the host DNA to be replicated.
Most fungal infections of the skin, scalp and nails are caused by Tinea, which is a dermatophyte fungus and spreads through the skin by small filaments called hyphae. Yeasts also cause fungal infection, the most common being Candida, which spreads by budding and infects mucous membranes of the mouth and vagina causing the infection candidiasis commonly referred to as thrush. The fungus malassezia is a yeast normally found in the skin but can overgrow and cause problems like dandruff. It also causes pityriasis versicolor, which appears as white to brown scaly patches on the body. Symptoms of a fungal infection include rash, itching, scaling of the skin and inflammation.
Most anti-fungal treatments have a broad-spectrum of action and work in the same way. They target a specific fungal enzyme needed produce ergosterol, which is a major component of the fungal cell membrane, and as a result the fungal membrane becomes weakened and leaks. This kills the fungus and prevents the infection. Anti-fungals are available as tablets and capsules for oral administration, gels and creams for topical infections and shampoos for infections of the scalp.