Viruses are distinct from bacteria, these are very small particles which contains a nucleic acid molecule in a protein coat. It is not visible under the microscope and do not have a cell wall like bacteria. They can only replicate within the living cells of the host and takes over the function of the cell or “parasitize” the cell. Viruses are the most abundant “living things” in the universe that we know and its origins are as old as the universe itself.
Viruses are the most ubiquitous living things and they are found in every corner of the world as long as there are living things to parasitise. Some viruses predominate in certain geographic locations but soon spread to other regions due to human migration, travel and animal migration. Zika virus was recently spread from South America to Europe and Asia after being brought back by travellers. Certain viruses like West Nile viruses are more confined to geographic locales. HIV virus probably started in Africa and spread in the 1980’s and now affects populations all over the world. Certain populations are more prone to specific viruses due to genetic, environmental and lifestyle factors e.g. Hepatitis A in the Eskimo population, Hepatitis B in certain areas of Taiwan and China, Ebola virus in Central Africa, etc. Dengue and Chikungunya virus are found in the tropics between latitude 23 degrees north and south of the Equator. Rabies virus is maintained in certain animal reservoirs and thus confined to areas where such animals live. The same example is true for Japanese B Encephalitis virus.
Some viruses can spread rapidly throughout the world and the virus that causes such pandemics is the influenza virus. Influenza causes the seasonal flu outbreaks which are usually worse in the winter in temperate countries. When there is sufficient change in the protein structure (antigenic shift) of the influenza virus, pandemics may result. Other viruses that have caused pandemics include SARS-CoV and others that have “pandemic potential” include MERS-CoV, avian influenza etc. Other viruses that have caused recent large regional outbreaks that have captured global attention include dengue, Ebola etc.
Viruses are transmitted by the following routes :
- Touch /close contact : Herpes, enterovirus, influenza virus, etc
- Airborne : respiratory viruses
- Exchange of body fluids (including sexual ): CMV, EBV, sexual contact (herpes, venereal warts , HIV, Ebola, Zika)
- Transfusion : HIV, CMV, Hepatitis B , mother to child transmission
- Fecal-oral : foodborne viruses
- Insect or vector borne : Dengue, Zika, Chikungunya, yellow fever, Japanese B encephalitis, West Nile virus, etc
- Animal bites : Rabies, Lyssa virus, simian herpes virus, etc
- Organ transplantation : HIV, West Nile virus, hepatitis B, C, etc
The clinical syndromes may be broadly divided into the following :
- Cutaneous disease ( with /without mucosal/oral involvement ) : The common viral skin diseases are viral warts (human papilloma virus ), molluscum (poxvirus ), cold sores or orolabial herpes (usually herpes simplex virus type 1), genital sores (usually herpes simplex virus type 2), chickenpox (varicella zoster virus ), shingles (varicella zoster virus ) and slapped cheek disease (fifth disease in children caused by Parvovirus B19 )
- Gastrointestinal viral infection : Usually presents with acute onset of fever, vomiting, diarrhoea and stomach cramps eg “food poisoning” / “gastric flu”. Common viruses involved are Rotavirus (mainly children), Norovirus, (aka Norwalk), enterovirus, etc.
- Hepatitis : Jaundice is usually a sign of liver disease and may be caused by hepatitis virus infection(Hepatitis A, B, C,D, E )
- Respiratory syndrome : They can present as a mild flu-like illness such as rhinitis or can involve the lung such as pneumonitis and develop into severe respiratory disease ( ARDS ) like SARS. Common cold virus (rhinovirus ) usually present with rhinitis and more severe respiratory infection from influenza usually presents with fever, headaches, bodyaches and prostration. CMV and EBV viral infection can present as an acute pharyngotonsillitis with rash and lymphadenitis (Infectious Mononucleosis Syndrome ). Sometimes, viral pneumonias including influenza, adenovirus and coronavirus (SARS CoV and MERS CoV ) can progress rapidly and result in acute respiratory failure requiring mechanical ventilation and ECMO ( extra-corporeal membrane oxygenation using heart lung machine )
- Brain syndrome ( meningoencephalitis ) : The important viral encephalitides are Enteroviruses encephalitis, Herpes simplex viral encephalitis, Japanese B Encephalitis, Yellow Fever encephalitis, Rabies encephalitis and rarely Dengue encephalitis
- Viral rash syndrome (usually associated with fever ): Measles, Dengue, Zika, Epstein Barr Virus ( IMS ), Parvovirus B19, hand foot mouth disease (enterovirus )
- HIV infection : this infection can have a varied clinical presentation and can be asymptomatic, detected only by screening tests (details are in the separate section on HIV infection on our website)
Certain viruses which are common in healthy individuals may present with severe invasive disease in the immunocompromised eg in HIV patients, transplant patients, chemotherapy patients or pregnant women.
Of special mention is a group of viruses that can cause fetal infection and developmental anomalies in the fetus as a result of intrauterine infection. These are Rubella virus, chickenpox virus, Zika virus, cytomegalovirus and herpes virus. In addition, viruses like HIV , hepatitis B, Dengue , enteroviruses can be transmitted via placental circulation in late pregnancy or during delivery , causing infection of the newborn.
- Hepatitis B and C predispose to liver cancer
- Epstein Barr virus and nasopharyngeal cancer, lymphoma
- Epstein Barr virus and nasopharyngeal cancer, lymphoma
- Human herpes virus 8 and Kaposi’s Sarcoma
- HIV and various cancers
- HTLV1 and lymphoma
Most of the viruses causing cutaneous disease can be diagnosed clinically ie by typical visual appearance eg viral warts, herpes simplex virus, shingles, chickenpox and measles. in atypical presentations, virus isolation can be used to confirm the diagnosis eg. vesicle fluid, cerebrospinal fluid, saliva swabs, nasopharyngeal secretions etc.
Respiratory viral infections are best confirmed by running tests on a respiratory tract sample such sputum or nasopharyngeal aspirate. These tests include PCR ( polymerase chain reaction ) for multi-plex respiratory viruses. Other viruses may also be detected in the blood. Serology testing are also done in other viruses eg. Dengue, Zika ( urine test ), chikungunya virus and hepatitis viruses to confirm he diagnosis.
In general, antibody tests should be done at least one week after the onset of illness as such tests may not be reactive if done too early and may result in false negative tests. The same is said for HIV antibody test and we are aware of the perils of testing during the “window period”.
For most viral infections, it is your own host immune system that finally controls the infection. Hence, the “usual” assessment and recommendation by doctors that most viral infections are self-limiting. However, in certain individuals , treatment may be necessary eg. in HIV patients and immunocompromised patients or those with severe and complicated infections.
Some anti-viral drugs if started early can mitigate disease course and result in a shorter or milder illness and better outcomes eg .
- Tamiflu for influenza
- Valacyclovir for chickenpox and herpes simplex virus infection
- Anti-viral for herpes zoster infection
- CMV ( cytomegalovirus ) infection in immunocompromised hosts
- Antiviral treatment for herpes encephalitis/meningitis
- Anti-HIV treatment
Many viral infections can be prevented by simple hand hygiene and food/water hygiene practice. These are viruses that are borne by hands, food and water.
Vaccination has been a very effective tool in the control of viral diseases. Smallpox has been eradicated by a successful vaccination program that started in the 1950s. The same strategy has been employed for the near eradication of polio viruses. There are still pockets of polio transmission sporadically. Some of the effective antiviral vaccines are :
- MMR ( Measles Mumps Rubella vaccine )
- Hepatitis B vaccine
- Hepatitis A vaccine
- Chickenpox vaccine
- Human Papilloma Virus vaccine
- Influenza vaccine
- Rotavirus vaccine
- Rabies vaccine
- Japanese B encephalitis vaccine
- Yellow Fever vaccine
Newer vaccines include herpes zoster vaccine and Dengue vaccine. The search for an effective Zika and Ebola vaccine is now very close but we still have a long way to go for an effective HIV vaccine.
Types of Infections
- Adult Immunisation
- Antibiotic Therapy
- Bloodstream Infections
- Bone and Joint Infections
- Central Nervous System (CNS) infections
- Chronic Fatigue Syndrome
- Dengue Fever
- Fever (Pyrexia) Of Unknown Origin
- Fungal Infection
- HIV Infection / AIDS
- Infection In Implants And Devices
- Infections Associated With Organ Transplants
- Infections In Pregnancy
- Infections Related To Impaired Immunity
- Infective Endocarditis
- Latent Tuberculosis
- Mycoplasma Infections
- Parasitic Infections
- Sexually Transmitted Infections
- Travel Related Illnesses With A Focus On Infections
- Tropical Infections
- Typhoid Fever & Paratyphoid Fever
- Urinary Tract Infections
- Viral Infections