An asymptomatic carrier (or carrier), is a person who is infected with the agent of an infectious disease, or carries the abnormal gene of a recessive genetic disorder. Although unaffected themselves, a carrier can transmit it to others.
Mary Mallon, known as "Typhoid Mary", was an asymptomatic carrier of typhoid fever. She worked as a cook for several families in New York City approximately a century ago. Several cases of typhoid fever in members of those families were traced to her by the Health Department. It appeared that she "carried" the infectious agent without becoming sick. There was no way of eradicating the disease and an attempt was made to restrict her from continuing to work as a cook to avoid spreading it to others.
The daughters of Queen Victoria, the princesses Alice and Beatrix, were asymptomatic carriers of the X-linked hemophilia gene (more precisely, an abnormal allele of a gene necessary to produce one of the blood clotting factors). Both had children who continued to pass the gene to succeeding generations of the royal houses of Spain and Russia, into which they married. Males who carried the gene had hemophilia, while females simply passed it to about half of their children. The genetic term for this type of "carrying" of a recessive trait is heterozygote.