Important viruses and their properties


Group Virus Characteristics Clinical importance
Retroviruses Human T-cell leukaemia virus(HTLV-1)












Human immune deficiency virus (HIV)




Spherical enveloped virus, 100nm in diameter

Icosahedral cores contain two copies of linear RNA molecules and reverse transcriptase.




Differs from other retroviruses in that core is cone-shaped rather than icosahedral.

HTLV is spread inside infected lymphocytes in blood, semen or breast milk.

Most symptoms remain asymptomatic but after an incubation period of 10-40 years in about 2% cases, adult T-cell leukaemia can result.







Blood or genital secretions are the source of transmission.

The principal target for the virus is the CD4+ T- lymphocytes cells.

Depletion of these cells induce immunodeficiency.




Mumps virus






Measles virus




Enveloped particles variable in size.

110-170nm in diameter with helical capsids.



Enveloped particles variable in size, 120-250nm in diameter with helical capsids.

 Swelling of parotid and sub-maxillary salivary glands in case of children.

Neurological complications like meningitis especially in adults might be seen.


Very common in children with characteristics fever.

Life-long immunity with rare case of second attack.

Rhabdovirus Rabies virus Bullet shaped particles, 75-180nm, Enveloped, helical capsids. Wide host range infecting all mammals.

Dogs, cats and cattles are particularly susceptible.

Incubation period ranges from 6 days to 1 year.

Remains localized in a wound for a while before passing along the nerve fibres to CNS.

Fatal encephalitis might be noticed.



Polio virus















Naked icosahedral particles having 28nm diameter.





Naked icosahedral, 30nm in diameter.






Naked icosahedral, 27nmin diameter.

One of a group of Enteroviruses common in the gut of humans.

Lymphoid tissue of alimentary tract is the primary site of multiplication.

Rare case of systemic infections or serious neurological conditions like encephalitis or poiliomyelitis.


Common cold viruses with over 100 antegenically distinct types.

Difficulty in preparing effecting vaccines.

Nasal secretions shed copious virus.


Cause infectious hepatitis in children through oral-faecal route.

Associated with sewage contamination of food and water supplies.

Reoviruses Rotavirus Two concentric icosahedral shells surround an inner core.

70nm in diameter.

Cause of gastroenteritis in infants.

Poor water supplies and low level of general hygiene are the causes.

Responsible for million deaths each year in developing countries.

Hepatitis viruses Hepatitis-B virus Spherical enveloped particle of size 42 nm in diameter.

Encloses an inner icosahedral 27nm nucleo-capsid.

Most children are infected by perinatal transmission in South East Asia and Africa.

Contact with contaminated blood and sexual intercourse is the cause for spread in Western world.

Chronic infection with HBV can progress to liver cancer.

Adenoviruses Adenovirus Icosahedral particles of 80nm in diameter  Common cause of upper respiratory tract infections.

Tend to produce latent infections in tonsils and adenoids.

Produce tumors if injected into hamsters, rats or mice.

Poxviruses Variola/Vaccinia Large particles 200-250nm, complex symmetry. Variola is the small pox virus which produces a systemic infection with vesicular rash on face, arms and legs with a high mortality rate.

Vaccinia has been derived from the cowpox virus which is used to immunize against smallpox virus.

Papovaviruses Papilloma virus Naked icosahedral

50nm in diameter.

Cause of warts which multiplies only in epithelial cells of skin and mucous membranes.

Some types might be associated with cervical carcinoma.

Myxoviruses Influenza virus(RNA virus) Enveloped particles.

100nm in diameter with a helically symmetric capsid.

Haemagglutinin and neuraminidase spikes project from the envelope.

Capable of extensive antigenic variations.

Human immunity becomes ineffective against new antigenic type which results in pandemics of influenza.

The virus only multiplies in the cells lining the upper respiratory tract in natural infections.

Absorption of toxic breakdown products from the dying cells on the respiratory epithelium might result in constitutional symptoms.

Flaviviruses Yellow fever virus




Hepatitis-C virus

Spherical particles 40nm in diameter with an inner core surrounded by an adherent lipid envelope.


Spherical particles 40nm in diameter consisting of an inner core surrounded by adherent lipid envelope.

Spread to humans by mosquito bites.

Target organ is liver.

Necrosis of hepatocytes that may result in jaundice and fever.

Blood transfusions and blood products are the sources of transmission.

Induces hepatitis which is usually milder than that caused by HBV.

Filoviruses Ebolavirus Long filamentous rods composed of lipid envelope surrounding a helical nucleocapsid 1000nm long, 80nm in diameter.


Widespread to population of monkeys.

Can be spread to humans by contact with body fluids from the primates.

The resulting haemorrhagic fever has a 90% case fatality rate.

Hepatitis viruses Hepatitis-D virus (HDV) An RNA containing virus that can only replicate in cells co-infected with HBV.

The spherical coat of HDV is composed of HBV capsid protein.

The presence of the satellite HDV exacerbates the pathogenic effects of HBV producing severe hepatitis.
Herpesviruses Herpes simplex virus (HSV1 and HSV2)





Varicella-zoster virus (VZV)








Cytomegalovirus (CMV)









Epstein-Barr virus (EBV)

Enveloped, icosahedral particles 150nmin diameter.





Enveloped, icosahedral particles 150nm in diameter.







Enveloped, icosahedral particles 150nm in diameter








Enveloped, icosahedral particles 150nm in diameter.

HSV1 infects oral membranes in children.

More than 80% are infected by adolescence.

Can have 50%chance of developing cold sores.

Virus DNA gets retained in the trigeminal nerve ganglion for life following the primary infections.

HSV2 is responsible for recurrent genital herpes.



Cause chicken pox in children.

Remains dormant in any dorsal root ganglion of the CNS.

Release of immune control in the elderly stimulates reactivation resulting in shingles.



Acquired as subclinical infection in childhood.

50% of adult carry the virus in dormant state in white blood cells.

Causes severe diseases like pneumonia, hepatitis, encephalitis in immune-compromised patients.

Primary infections during pregnancy can induce serious congenital abnormalities in the fetus.


Salivary exchange results in infections.

Asymptomatic in case of young children but persists in latent form in lymphocytes.

Infection delayed Until adolescence often results in glandular fever.

In tropical Africa, a severe EBV infection early in life predisposes the child to malignant facial tumors (Burkitt’s lymphoma).






Important viruses and their properties