Opportunistic infections are infections that occur more frequently and are more severe in individuals with weakened immune systems. These infections take advantage of the body's reduced ability to fight off pathogens, often affecting people with conditions like HIV/AIDS, cancer, or those taking immunosuppressive medications. Common opportunistic infections include Pneumocystis pneumonia, tuberculosis, and certain types of fungal infections. They can be life-threatening if not diagnosed and treated promptly. Preventive measures, such as vaccinations and prophylactic medications, are essential for at-risk individuals to reduce the likelihood of these infections.