It is possible that there are as yet unidentified hazards in exposure to viruses. Individuals may become infected with viruses in the laboratory, as elsewhere, by inhalation, ingestion, inoculation, or other direct contact. These individuals may then develop symptoms or may remain for a time nonsymptomatic carriers. In either case, they may transmit the virus infection to others such as family members by direct contact or indirectly by shedding viruses and contaminating the environment. Infants and young children are particularly susceptible to virus infections.
Some virus infections are more virulent when they occur in pregnant women.
Viruses may cross the placenta to the fetus and may result in miscarriage, fetal death, fetal infection, or developmental abnormalities. In children they may cause upper respiratory infection, croup, bronchitis, broncheolitis, anc bronchopneumonia. Pneumonia in childhood is the most serious condition and may be fatal. Nine strains have been identified as causing tumors in newborn hamsters. Most individuals have been infected with Adenovirus by the age of 15 and greater than 80 percent of the individuals have antibodies to Adenovirus.