What pathogens can people be natural carriers of?

Natural Carriers of Pathogens in Humans

Bacterial Carriers

Staphylococcus aureus (Nasal Carriage)

Approximately 30% of the healthy human population are persistent nasal carriers of Staphylococcus aureus, with an additional 60% being intermittent carriers, making this one of the most common natural carrier states. 1, 2

• The anterior nares serve as the primary ecological niche for S. aureus colonization 2
• Three distinct carriage patterns exist: persistent carriers (20%), intermittent carriers (60%), and non-carriers (~20%) 2
• Carrier strains have evolved mechanisms to suppress innate immunity, delaying host antimicrobial responses and allowing preferential colonization 3
• The human factor itself is a critical determinant—persistent carriers typically return to their original carrier state even after artificial inoculation with different strains 4

Coagulase-Negative Staphylococci

• Staphylococcus epidermidis commonly colonizes human skin and nasal passages 5
• S. lugdunensis can cause native valve infections despite being a natural colonizer 6
• These organisms frequently cause device-related infections through biofilm formation 6

Streptococcal Species

• Viridans streptococci colonize the oral cavity and are found in approximately 80% of human bite wounds 7
• Streptococcus pneumoniae colonizes the nasopharynx and can cause respiratory infections, particularly in children, elderly, and immunocompromised individuals 6

Oral Flora

• Eikenella corrodens is found in approximately 30% of human bite wounds, representing normal oral flora 7
• Anaerobes including Fusobacterium species, peptostreptococci, Prevotella species, and Porphyromonas species colonize the oral cavity and are present in about 60% of human bite infections 7
• Corynebacterium species are normal skin and mucosal colonizers 6
• Propionibacterium acnes colonizes the nasal passages and skin 5

Respiratory Pathogens

• Bordetella pertussis can be carried asymptomatically, with mothers, fathers, and grandparents serving as sources of infant infections in 39%, 16%, and 5% of cases respectively 6
• Neisseria meningitidis can colonize the nasopharynx without causing disease 6

Viral Carriers

Respiratory Viruses

• Rhinoviruses, human coronaviruses, respiratory syncytial virus (RSV), and adenoviruses cause 30-80%, 15%, 10-15%, and 5% of common colds respectively, with asymptomatic carriage possible 6
• Influenza virus can be transmitted during asymptomatic or pre-symptomatic phases 6

Herpes Viruses

• Herpes simplex virus (HSV) establishes latent infection and can be transmitted through close contact, including during athletic activities 6
• Epstein-Barr virus requires close contact for transmission and can cause endemic disease in adolescent group settings 6

Bloodborne Viruses

• Hepatitis B, hepatitis C, and HIV can be carried chronically, though transmission during athletic competition has not been validated 6

Fungal Carriers

• Candida species colonize mucosal surfaces and can cause opportunistic infections 6
• Tinea species (T. capitis, T. corporis, T. pedis, T. cruris) colonize skin and are transmissible through skin contact 6
• Aspergillus niger can be found in the environment and occasionally colonizes humans 6

Parasitic Carriers

• Pediculosis (capitis, corporis, pubis) represents ectoparasitic carriage transmissible through close contact 6
• Free-living amoebae can carry bacterial pathogens like legionellae 6

Important Clinical Considerations

Risk Factors for Increased Carriage

• Healthcare workers have three times higher risk of tuberculosis carriage compared to the general population 6
• Patients with HIV infection, those on hemodialysis or peritoneal dialysis, and intravenous drug users show increased S. aureus carriage rates 2
• Hospitalized patients show enriched S. aureus or S. epidermidis colonization with diminished actinobacterial groups 5

Transmission Dynamics

• Spontaneous bacteremia from normal activities like chewing and tooth brushing occurs frequently, explaining why most infective endocarditis cases are unrelated to invasive procedures 6
• Healthcare devices, environmental surfaces, and transfer between staff and patients serve as sources for pathogen acquisition 6
• Close contact in athletic settings facilitates transmission of skin, respiratory, and gastrointestinal pathogens 6