An opportunistic infection is an infection that occurs most commonly in individuals with an immunodeficiency disorder and acts more severely on those with a weakened immune system. These types of infections are considered serious and can be caused by a variety of pathogens including viruses, bacteria, fungi, and parasites. Under normal conditions, such as in humans with uncompromised immune systems, an opportunistic infection would be less likely to cause significant harm and would typically result in a mild infection or no effect at all. These opportunistic infections can stem from a variety of sources, such as a weakened immune system, when being treated with immunosuppressive drugs, when a microbiome is altered, or when integumentary barriers are breached. Opportunistic infections can contribute to antimicrobial resistance in an individual making these infections more severe. Some pathogens that cause these infections possess intrinsic resistance to many antibiotics while others acquire resistance over time through mutations or horizontal gene transfer.