Infection is invasion, multiplication, and the body's heated reply
An infection begins when pathogens enter tissues, multiply, and provoke the host to react against them and their toxins. The resulting illness is an infectious disease, and some of these also spread between people. Bacteria and viruses lead the cast, but fungi, parasites, viroids and even prions join in.
Being infected and being sick are not the same thing. Some infections never make their host ill, and certain diseases stay silent for their whole course in one person, counting as a disease only in the people that carrier goes on to infect. When symptoms do appear they may be general, such as fever, chills, night sweats, tiredness, weight loss and aches, or local, like a rash, a cough or a streaming nose. Because bacteria and viruses can produce identical symptoms, telling them apart matters: antibiotics cure bacterial infections but do nothing against viruses.
The list of culprits is long. Among bacteria are the agents of tuberculosis and botulism alongside Staphylococcus aureus, E. coli and Salmonella. Viruses range from the common cold's rhinovirus to HIV, rabies, Ebola and SARS-CoV-2. Fungal infections include Candida yeasts, the most common kind, plus Aspergillus, Pneumocystis, Cryptococcus and the skin-dwelling dermatophytes. Parasites split into single-celled organisms such as those behind malaria and toxoplasmosis, and worms including roundworms, tapeworms and schistosome flukes. Ticks, lice, mites and fleas cause comparable harm, though doctors usually call their presence an infestation.
Infection unfolds as a chain: an agent, a reservoir where it lives, a way out, a route of transmission and entry into a susceptible host. Every link has to occur in order, which is why breaking any one of them prevents disease. Microbes usually get in through mucous membranes of the mouth, nose, eyes, genitals or anus, or through wounds, then settle and multiply in a step called colonisation. Some stay near the entry point, but many travel to other organs; some live inside cells, others float free in body fluids. Most healthy people resist easily, while those with weakened immunity are prone to persistent and opportunistic infections.
Mammals respond first with a fast innate defence, often inflammation, followed by a slower adaptive one. Treatment depends on the invader, with anthelmintic drugs used against worms, for example. The burden remains heavy: in 2017 communicable, maternal, newborn and nutritional conditions together caused an estimated 18.6 percent of deaths worldwide.
Source: Infection
