Infection

An infection is the entry and establishment of a small living organism—such as bacteria, virus, fungus, or parasite—within a host body. These organisms, called pathogens, may damage tissue, produce toxins, or trigger a strong immune response. The term 'infection' sometimes refers only to the presence of the pathogen, even without symptoms.

Infection and disease are not the same. A person can carry a small organism without becoming ill; this is called asymptomatic infection. When symptoms such as fever, pain, or inflammation appear, the infection is recognized as disease. Public health distinguishes between the two because an unaffected carrier can still transmit the organism.

Types of pathogens

Bacteria are single-celled organisms that reproduce on their own; examples include those causing diarrhea and some skin infections. Viruses are extremely small and cannot reproduce without entering a living cell; they cause influenza, measles, and many others. Fungi typically affect the skin, nails, and mucous membranes, particularly in people with weakened immunity. Parasites include single-celled organisms such as malaria parasites and intestinal worms. Each type requires different treatment: antibiotics work against bacteria, but do not kill viruses.

Routes of transmission

Transmission occurs through air (coughing and sneezing), contaminated food and water, contact with infected skin or wounds, sexual contact, blood, unsterilized medical equipment, and animals such as mosquitoes and ticks. Some pathogens pass from mother to fetus or during birth. Understanding the transmission route determines prevention: clean water, hand washing, insect nets, or quarantine.

Immune response

The body has two levels of defense. Innate immunity—the skin, stomach acid, and cells like macrophages—launches an immediate attack; these are fever and inflammation. Adaptive immunity produces T cells, B cells, and antibodies targeting specific pathogens, then remembers them for rapid response if they return. Vaccination works by training this memory without causing disease.

Diagnosis, treatment, and prevention

Diagnosis relies on symptoms, blood tests, cultures of the organism, and tests detecting pathogen markers. Treatment depends on the type: antibiotics, antivirals, or antifungals; sometimes only supportive care such as fluids and rest is given. Inappropriate antibiotic use leads to bacterial resistance, making future treatment harder. Prevention depends on sanitation, vaccination, clean water and nutrition, and public health surveillance.