How do viruses transmit from host to host?

Lysogeny in prokaryotes

• The relationship between the temperate phage and the host bacterium.
• The host bacterium is the lysogen and the phage within the lysogen is the prophage.
• The prophage may be integrated into the host genome, or remain free in the cytoplasm.
• Lysogeny can change the lysogen phenotype: this is called lysogenic conversion.
• Infection in eukaryotes is categorized: acute, latent, chronic and transformative (into malignant cell).

Transmission from one host to another

• Occurs by:
• Aerosol transmission
• Faecal-oral transmission
• Direct contact of body fluids
• Zoonotic transmission
• Vectors - mosquito
• Some viruses replicate at the site of entry, yet some may enter the bloodstream (viremia) and spread to target tissues. In addition the virus progeny can release into the blood (secondary viremia) for infection of further sites.

Airborne disease

• Examples:
• Flu - Acute respiratory disease, featuring high fever, cough, headache and loss of appetite. Seasonal epidemics occur and pandemics have been recorded. Effects unbiasedly, however those immunocompromised are most susceptible.
• Chickenpox - Acute childhood disease presents as a rash and progresses to blisters. The virus can remain latent in the body and cause shingles in adults.
• Measles and Rubella - Acute childhood disease characterised by body rash. Enters via the respiratory tract, multiplies then is disseminated through the body.
• Mumps - Acute childhood disease characterised by swollen salivary glands. Virus enters via respiratory tract.

Food and waterborne disease

• Hepatitis A & E - Acute disease
• Viral gastroenteritis - Acute disease
• Poliomyelitis - May be acute or latent, latent resulting on an attack on the NS, leading to paralysis.

Direct contact diseases

• AIDS - acute and chronic disease. Acute phase infection of CD4 T cells, resulting in brief illness. the virus disseminates into the lymphoid tissues and the patient becomes asymptomatic for up to 10 years. Chronic phase is characterised by massive depletion of CD4, recurrence of generic illness symptoms. AIDS follows.
• Hepatitis B & C - All hepatitis presents in inflammation of the liver. B and C are acute or chronic diseases and may lead to liver cancer.
• Genital herpes & Cold sores - Herpes simplex 1 and 2. A classic example of acute and chronic infection that usually lasts a lifetime. Acute phase results in blisters, then the virus retreats to nerve cells during latency. Stress and hormonal changes can stimulate the virus.
• Warts - human papilloma virus.

Vector - Arthropod borne disease

• Acute infection, mostly transmitted by Aedes and Culex species of mosquito.
• Dengue fever and Zika virus are examples.

Zoonotic disease

• Associated substantial mortality rates.
• Viral hemorrhagic fever - Ebola. Damage to the entire vascular system, resulting in leakage. Most likely origin is primates or fruit bats. Transfer by handling and eating infected animals.
• Rabies - Acute, progressive inflammation of the spinal cord. Transmitted through bites (replicated in salivary gland). Initially infects skeletal muscles, then spreads to CNS. Always following clinical symptoms of hydrophobia, hyperactivity and excited behaviour.
• Avian flu (bird flu) H5N1 and H7N9 - Clinically distinct from influenza A H1N1 type virus. This is a lower respiratory tract infection, resulting in pneumonia and acute respiratory distress syndrome. High mortality rate of 30-50%. Transferred by consuming or handling undercooked poultry.

Viruses that cause cancer

• Categorized as oncoviruses. Contain viral oncogenes, producing suppressor proteins for the host tumor suppressor proteins.
• Examples are Hep B and C for liver cancer, human papillomavirus for cervical cancer.