A person carrying streptococcus in their mouth is not necessarily immune to streptococcal pharyngitis when in contact with someone who has it. While the presence of certain bacteria in the body can reduce the risk of infection, being a carrier does not guarantee immunity. The chances of contracting streptococcal pharyngitis may be somewhat reduced in a carrier, but it does not provide full protection. It is important to note that antibiotic use should be limited, as overuse can disrupt the body's natural balance of bacteria and potentially increase the risk of infections in the future.