What causes an infection in the body?

An infection in the body is primarily caused by the invasion of pathogens. Pathogens include microorganisms such as bacteria, viruses, fungi, and parasites that can enter the body and multiply. Once these pathogens invade, they can disrupt normal biological functions, trigger an immune response, and lead to various symptoms associated with infection, such as fever, inflammation, and fatigue.

In contrast, allergies are immune responses to substances (allergens) that are usually harmless, rather than infections caused by pathogens. Vitamin deficiencies may lead to weakened immune function or other health issues but do not directly cause infections. Environmental factors can contribute to the likelihood of infections (for example, contamination or exposure to sick individuals), yet they are not the direct cause of the infection itself.