Viral Diseases of Humans

Overview

This section summarizes key viral diseases relevant to human health, focusing on causative agents, transmission, clinical features, and prevention. Understanding these diseases is essential for microbiology students, especially in the context of epidemiology, immunity, and public health.

SARS, MERS, COVID-19, and the Common Cold

• Causative Agent: Coronaviruses; enveloped, single-stranded RNA viruses.

• Transmission: Droplet, direct contact, indirect contact.

• General Disease Info: Causes common cold and severe acute respiratory diseases.

• Unique Symptoms: Loss of taste/smell (notable in COVID-19).

• Vaccine: COVID-19 vaccines (mRNA-based, e.g., Comirnaty, SPIKEVAX).

• Zoonotic: Some are (e.g., SARS, MERS, COVID-19).

• Emerging Infectious Diseases: SARS, MERS, COVID-19.

Example: SARS-CoV-2, the agent of COVID-19, caused a global pandemic beginning in 2019.

Polio (Poliomyelitis)

• Causative Agent: Poliovirus (Picornaviridae); nonenveloped, ssRNA.

• Transmission: Fecal-oral route (contaminated food and water).

• General Disease Info: Most infections are mild; severe cases can result in paralysis.

• Vaccine: Inactivated polio vaccine (IPV) and oral polio vaccine (OPV).

• Unique Symptoms: Paralysis, especially in children.

• Eradication: Polio is targeted for global eradication.

Example: The Salk (IPV) and Sabin (OPV) vaccines have dramatically reduced polio incidence worldwide.

Hepatitis Viruses (A, B, C)

Hepatitis A

• Causative Agent: Hepatitis A virus (HAV); nonenveloped, ssRNA.

• Transmission: Fecal-oral (contaminated food/water), direct contact.

• Symptoms: Liver infection, dark urine, jaundice.

• Vaccine: Yes (Havrix, VAQTA).

Hepatitis B

• Causative Agent: Hepatitis B virus (HBV); enveloped, dsDNA (reverse transcriptase).

• Transmission: Blood and sexual contact.

• Symptoms: Jaundice, chronic hepatitis, hepatic cancer.

• Vaccine: Yes.

Hepatitis C

• Causative Agent: Hepatitis C virus (HCV); enveloped, ssRNA.

• Transmission: Blood-borne, less commonly sexual.

• Symptoms: Jaundice, chronic liver infection, risk of liver cancer.

• Vaccine: No.

Comparison Table:

Chickenpox (Varicella), Cold Sores, Genital Herpes

• Causative Agents: Herpesviruses (Varicella-zoster virus, HSV-1, HSV-2).

• Transmission: Direct contact, respiratory droplets, sexual contact (HSV-2).

• Symptoms: Chickenpox: vesicular rash; Herpes: cold sores, genital lesions.

• Vaccine: Yes (MMRV or Varicella vaccine).

• Latency: Viruses can remain latent and reactivate (e.g., shingles from VZV).

Example: Shingles (herpes zoster) is a reactivation of latent varicella-zoster virus.

HIV/AIDS

• Causative Agent: Human immunodeficiency virus (HIV); retrovirus, ssRNA, reverse transcriptase.

• Transmission: Blood, sexual contact.

• Symptoms: Immunodeficiency, opportunistic infections, progression to AIDS.

• Unique Features: Latency, integration into host genome, gradual immune system decline.

• Vaccine: No.

Example: HIV infection can remain asymptomatic for years before progressing to AIDS.

Human Papillomavirus (HPV)

• Causative Agent: Human papillomavirus; dsDNA.

• Transmission: Sexually transmitted.

• Symptoms: Warts, risk of cervical and other cancers.

• Vaccine: No (note: vaccines are available in practice, e.g., Gardasil, but not listed in the notes).

Example: Persistent HPV infection is a major risk factor for cervical cancer.

Influenza (Flu)

• Causative Agent: Orthomyxoviruses; segmented, negative-sense RNA, enveloped.

• Transmission: Respiratory droplets, contact.

• Symptoms: Fever, chills, muscle aches, cough.

• Vaccine: Yes (annual, trivalent or quadrivalent).

• Emerging Strains: Avian influenza, H5N1, H1N1 (swine flu).

Additional info: Antigenic drift and shift contribute to influenza pandemics.

Measles (Rubeola)

• Causative Agent: Paramyxovirus (Morbillivirus); ssRNA, enveloped.

• Transmission: Airborne, highly contagious.

• Symptoms: Koplik spots, maculopapular rash, fever.

• Vaccine: Yes (MMR or MMRV).

Example: Measles can cause severe complications, especially in young children.

Mumps

• Causative Agent: Paramyxovirus (Rubulavirus); ssRNA, enveloped.

• Transmission: Saliva, respiratory droplets.

• Symptoms: Parotitis (swelling of salivary glands), fever.

• Vaccine: Yes (MMR or MMRV).

Rabies

• Causative Agent: Lyssavirus; ssRNA, enveloped.

• Transmission: Animal bites (saliva), direct contact.

• Symptoms: Encephalitis, hydrophobia, neurological symptoms.

• Vaccine: Yes (pre- and post-exposure prophylaxis).

Example: Rabies is almost always fatal once symptoms appear.

Rubella (German Measles)

• Causative Agent: Rubivirus; ssRNA, enveloped.

• Transmission: Airborne, respiratory droplets.

• Symptoms: Mild rash, lymphadenopathy; can cause congenital rubella syndrome if infection occurs during pregnancy.

• Vaccine: Yes (MMR or MMRV).

West Nile Virus

• Causative Agent: Flavivirus; ssRNA, enveloped.

• Transmission: Vector-borne (mosquitoes).

• Symptoms: Most infections are mild; severe cases involve neurologic disease.

• Vaccine: No.

Example: West Nile encephalitis is a severe complication in some patients.

Key Concepts and Definitions

• Zoonotic Disease: Disease that can be transmitted from animals to humans.

• Emerging Infectious Disease: Disease whose incidence has increased in the past two decades or threatens to increase soon.

• Vaccine: Biological preparation that provides active acquired immunity to a particular infectious disease.

• Latency: Ability of a virus to remain dormant within a host cell and reactivate later.

• Notifiable Disease: Disease that must be reported to public health authorities.