The majority of human viral infections fall into the category of acute self-limiting infections. Optimum transmissibility is crucial, and with viruses that cause systemic infections with lifelong immunity, perpetuation is possible only in large, relatively dense populations.

Mechanisms of viral transmission

• Aerosol
• Food and water
• Fomites
• Body secretions
• Sexual activity
• Birth
• Transfusion/transplant
• Zoonoses (animals, insects)

Viral Shedding and Routes of Transmission

Transmission cycles require virus entry into the body, replication, and shedding with subsequent spread to another host.

Virus transmission may be horizontal or vertical; however, most transmission is horizontal, that is, between individuals within the population at risk.

Shedding: release of infectious viruses from infected host

• Respiratory secretions. e.g rhinoviruses, influenza viruses

–Aerosolization – sneezing, coughing

–Contamination of fomites by nasal secretions

• Saliva. e.g. mumps, cytomegalovirus, rbies

–Aerosolization – sneezing, coughing

–Contamination of fomites – spitting, coughing, wiping hands

–Kissing, grooming (animals)

–Animal bites

• Feces, e.g. enteric and hepatic viruses

–Poor sanitation, food contamination, sexual exchange

• Blood, e.g. sindbis viruses (West Nile), Denge virus, hepatitis, HIV

–Transmission by biting insects, during sex, childbirth, exposure to contaminated blood

• Urine (viruea)

–Hantaviruses, arenaviruses

• Semen

–HIV, some herpesviruses, hepatitis B

• Milk

–Mouse Mammary tumor virus, Mumps, CMV

• Skin lesions

–Poxviruses, HSV, varicella zoster, papillomaviruses, Ebola virus

Modes of transmission of human viral diseases

Factors that promote transmission

• Virus stability
• Virus in aerosols and secretions
• Asymptomatic shedding
• Ineffective immune response