A carrier is an infected person or animal who does not have apparent clinical disease but is a potential source of infection to others.
- Healthy or asymptomatic carriers: These are persons whose infection remains unapparent. For example, in poliovirus, meningococcus and hepatitis virus infections, there is a high carrier rate.
- Incubatory or precocious carriers: These are individuals or persons who excrete the pathogen during the incubation period (i.e. before the onset of symptoms or before the characteristic features of the disease are manifested). E.g. Measles, mumps, chickenpox and hepatitis.
- Convalescent Carriers: These are those who continue to harbor the infective agent after recovering from the illness. E.g. Diphtheria, Hepatitis B virus.
- Chronic Carriers: The carrier state persists for a long period of time. E.g. Typhoid fever, Hepatitis B virus infection