Major Viral Diseases: Agents, Transmission, Symptoms, and Vaccines

Viral Diseases: Overview

This section summarizes key viral diseases relevant to microbiology, focusing on causative agents, transmission routes, clinical features, and available vaccines. Understanding these diseases is essential for grasping principles of pathogenicity, epidemiology, and immunology.

SARS, MERS, COVID-19

• Causative Agent: Coronaviruses; RNA viruses, enveloped

• Transmission: Droplet, direct contact, indirect contact

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

• Vaccine: COVID-19 vaccines available

Polio (Poliomyelitis)

• Causative Agent: Enterovirus (poliovirus); RNA, non-enveloped

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

• General Disease Info: Usually affects children under 5 years; can cause irreversible paralysis

• Unique Signs/Symptoms: Irreversible paralysis

• Vaccine: Yes – oral polio vaccine (OPV/Sabin) or inactivated polio vaccine (IPV/Salk)

Hepatitis A

• Causative Agent: Hepatitis A virus; RNA, non-enveloped

• Transmission: Fecal-oral route; vehicle (contaminated food and water), food-hand

• General Disease Info: Liver infection; most people recover

• Vaccine: Yes

Hepatitis B

• Causative Agent: Hepadnavirus (Hepatitis B virus – HBV); uses reverse transcriptase to make DNA from RNA

• Transmission: Direct contact (blood and semen)

• General Disease Info: Liver infection; can cause jaundice, chronic hepatitis, and liver cancer

• Vaccine: Yes

Chickenpox (Varicella) & Herpes

• Causative Agent: HSV-3 (Varicella-zoster virus, causes chickenpox); HSV-1 and HSV-2 (Herpes simplex viruses); DNA, enveloped

• Transmission: Direct contact, highly contagious (chickenpox); sexually transmitted (genital herpes)

• General Disease Info: Chickenpox – rash starts on the trunk, fluid-filled blisters; Herpes – genital ulcers and sores

• Vaccine: Chickenpox – yes (MMRV or Varicella vaccine)

• Location of Latency: Varicella-zoster virus (dorsal root ganglia); HSV (sensory ganglia)

• Complications: Shingles (zoster) can occur later in life

HIV/AIDS

• Causative Agent: Lentivirus; RNA, produces DNA

• Transmission: Sexually transmitted, blood-borne, vertical (mother to child)

• General Disease Info: HIV infection can progress to AIDS; many opportunistic infections occur in AIDS stage

• Vaccine: None currently available

HPV (Human Papillomavirus)

• Causative Agent: Human papillomavirus; dsDNA

• Transmission: Sexually transmitted

• General Disease Info: Can cause warts and is associated with cervical and other cancers

• Unique Signs/Symptoms: Warts – location depends on infection site

• Vaccine: Yes

Influenza (The Flu)

• Causative Agent: Orthomyxovirus; RNA, enveloped

• Transmission: Mainly droplet transmission; fomite transmission possible

• General Disease Info: Influenza A and B cause human disease; A is most severe and can cause pandemics; B is seasonal; C is sporadic and less severe

• Vaccine: Yes for A & B; yearly vaccine recommended

Measles (Rubeola)

• Causative Agent: Morbillivirus; RNA, enveloped

• Transmission: Airborne (droplet); highly contagious

• General Disease Info: Causes severe disease, especially in children; can cause complications such as pneumonia and encephalitis

• Unique Signs/Symptoms: Koplik spots (white spots in mouth), rash starts at face and spreads down

• Vaccine: Yes, MMR or MMRV (12m – 12y)

Mumps

• Causative Agent: Paramyxovirus; RNA, enveloped

• Transmission: Direct contact, droplet

• General Disease Info: Causes swelling and pain in salivary glands; can lead to complications such as orchitis (testicular inflammation)

• Vaccine: Yes, MMR or MMRV (12m – 12y)

Rabies

• Causative Agent: Lyssavirus; RNA, enveloped

• Transmission: Direct contact (usually through bite of infected animal)

• General Disease Info: Causes encephalitis; initial symptoms are non-specific, but once neurological symptoms appear, disease is almost always fatal

• Unique Signs/Symptoms: Hydrophobia, neurologic signs

• Vaccine: Yes; Imovax and RabAvert – can be given pre- or post-exposure

Summary Table: Major Viral Diseases

Disease	Causative Agent	Transmission	Key Symptoms	Vaccine
COVID-19	Coronavirus (RNA, enveloped)	Droplet, contact	Respiratory symptoms	Yes
Polio	Poliovirus (RNA, non-enveloped)	Fecal-oral	Paralysis	Yes
Hepatitis A	HAV (RNA, non-enveloped)	Fecal-oral	Liver infection	Yes
Hepatitis B	HBV (DNA, enveloped)	Blood, semen	Liver infection, cancer	Yes
Chickenpox	Varicella-zoster virus (DNA, enveloped)	Droplet, contact	Rash, blisters	Yes
HIV/AIDS	HIV (RNA, retrovirus)	Blood, sexual	Immunodeficiency	No
HPV	Human papillomavirus (DNA)	Sexual	Warts, cancer	Yes
Influenza	Orthomyxovirus (RNA, enveloped)	Droplet	Fever, cough	Yes
Measles	Morbillivirus (RNA, enveloped)	Droplet	Rash, Koplik spots	Yes
Mumps	Paramyxovirus (RNA, enveloped)	Droplet	Swollen glands	Yes
Rabies	Lyssavirus (RNA, enveloped)	Bite	Encephalitis, hydrophobia	Yes

Key Concepts

• Viral Structure: Viruses can be enveloped or non-enveloped; genetic material may be RNA or DNA.

• Transmission Routes: Include droplet, direct contact, sexual, blood-borne, and fecal-oral.

• Vaccines: Many viral diseases are preventable by vaccination (e.g., polio, measles, hepatitis B, influenza).

• Unique Symptoms: Some viruses have pathognomonic signs (e.g., Koplik spots in measles, hydrophobia in rabies).

• Chronicity and Cancer: Some viruses (e.g., HBV, HPV) can cause chronic infection and are associated with cancer.

Additional info:

• For HIV/AIDS, the transition from HIV infection to AIDS is defined by a drop in CD4+ T cell count and the appearance of opportunistic infections.

• Reverse transcriptase is a key enzyme in retroviruses (e.g., HIV, HBV) that allows RNA to be converted into DNA.

• Vaccines for rabies can be administered after exposure due to the long incubation period of the virus.