infection
What are types of infectious organisms?
How do pathogens spread from one host to another?
What are some examples of bacterial infections?
What measures can reduce the risk of infection?
What is vector-borne transmission?
infection, the process by which harmful microorganisms such as bacteria, viruses, fungi, or parasites enter cells and tissues in the body and cause disease. Infection may be localized or systemic (spread throughout the body), and its outcome depends on both the type of pathogen and the host’s immune response.
For more information about infectious diseases and their prevention, see:Types of infectious organisms
Although many bacteria are harmless or essential to health, certain species, referred to generally as pathogenic bacteria, are capable of causing infections. Examples of pathogenic bacteria include Escherichia coli, Staphylococcus aureus, and certain species of Streptococcus. Infection with these organisms produces illnesses that range from gastroenteritis to strep throat to life-threatening pneumonia or sepsis. Viral infections, in which pathogenic viruses enter and replicate inside host cells, often spread rapidly through populations; examples of such viruses include various influenza viruses, human immunodeficiency virus (HIV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Fungal infections include both superficial conditions, such as athlete’s foot and ringworm, and more serious systemic conditions such as candidiasis, which is particularly common in immunocompromised individuals. Parasitic infections occur when protozoa, helminths, or ectoparasites, such as the malaria parasite (Plasmodium), tapeworms, or lice, inhabit the body and draw resources from their host, sometimes leading to chronic illness.
Transmission
Pathogens can spread from one host to another in several ways. Airborne transmission occurs when infectious particles travel through droplets or aerosols, as seen in tuberculosis or influenza. Direct contact transmission takes place through physical touch, sexual contact, or exposure to bodily fluids, whereas indirect transmission involves contact with contaminated surfaces, food, or other objects, such as medical instruments. Some infections spread via vector-borne transmission, in which insects or other animals—such as mosquitoes carrying malaria or ticks carrying Lyme disease—transfer pathogens between hosts. Other modes include vertical transmission, in which infection is passed from mother to child during pregnancy, childbirth, or breastfeeding, and transfusion-transmitted infection, in which a pathogen is passed to the recipient of a blood transfusion.
Prevention and control
Effective measures to reduce the risk of infection include maintaining proper hygiene, ensuring safe food and water supplies, and following guidelines for practices such as handwashing and vaccination. Medical interventions with agents such as antibiotics, antivirals, antifungals, and antiparasitic drugs are essential for treating infections. Public health strategies, including surveillance, isolation or quarantine during outbreaks, and vector control, also play critical roles in limiting transmission. Together, these approaches form the foundation of modern infectious disease prevention and management.