Upper respiratory infections are contagious to some extent, as they are mostly caused by common pathogens such as rhinoviruses, streptococci, bacteria, mycoplasma, and other infectious agents. Clinically, they encompass a variety of diseases, including acute rhinitis, sinusitis, pharyngitis, tonsillitis, bronchiolitis, pneumonia, etc. These infections often spread through close contact and airborne droplets. Whether an individual gets infected mainly depends on the body's immune response. If a person has been vaccinated against a contagious disease and has developed antibodies, they generally will not be infected.