The epidermis of the skin is poorly vascularized and viruses introduced there tend to establish local infections. However, once the virus gets into the dermal layer of the skin, it accesses the blood vessels and can spread. The respiratory tract has complex host defense mechanisms that inhibit virus entry. The goblet cells secrete mucus, which is propelled by ciliated epithelia cells to clear foreign bodies by continuous upward movement. Humoral and cellular immune system also act to protect the delicate respiratory system.
Viruses enter by aerosolization.
E.g influenza viruses, parainfluenza viruses. To initiate infection via the GI route, a virus needs to be acid stable, resist loss of infectivity due bile salts and proteolytic enzymes. Other mucosal surfaces may allow virus entry. Sexual activity may produce tears or abrasions in the vaginal epithelium or trauma to urethra allowing viral entry. Sexually transmitted viruses include HIV, HSV, and papillomaviruses.