The placenta links the blood supply of a mother to that of her foetus to allow nutrients and oxygen to pass to the baby.
Viruses and antibodies also travel across the placenta from the mother to the baby.
If you become infected with any of the more common viruses - such as flu and chickenpox - when you are pregnant, they can be passed to your baby. This can be completely harmless to the baby, or may potentially have more serious effects. It depends on a number of things such as your own natural immunities, the stage of development of the foetus, and the type of virus.
There are some treatments available which help to prevent viruses being passed to the baby. If a mother carrying the Hepatitis B virus has her baby immunised in the first few hours after birth, it may not develop the infection. A woman with genital herpes lesions can prevent the herpes simplex virus being passed on to her baby by having a caesarean section or treatment with an antiviral medicine.