What is a virus? 13 common viruses and the diseases they cause

  • Wellness
  • Thursday, 16 Apr 2020

Many of the viruses on this list are transmitted through droplets expelled when an infected person coughs or sneezes. — Photos: TNS

All of us are probably familiar with the word “virus”, but how many of us actually know what a virus is and how it acts?

A virus is an obligate intracellular parasite, which means that it can only reproduce inside a host cell, using the cell’s resources.

They are unable to multiply by mitosis, or cell division, like bacteria do.

Most viruses range in size from 20 to 250 nanometres, and can only be seen through an electron microscope.

Three main properties distinguish viruses from other microorganisms: their small size, their genome (which is formed of either RNA or DNA, but not both) and their total dependence upon another living cell.

Viruses are named based on the disease they cause, their discoverers, geographical locations or how they were originally thought to be contracted.

They can enter our body through droplets in the air that we breathe in; droplets or materials on surfaces that we transfer to our mouth, eyes or nose; blood; sex; and animal or insect bites.

For most normal viral infections, no medicine is needed as our body’s immune system is enough to kill them.

We must remember that antibiotics are only effective for bacterial infections, not for viral infections.

Here are some of common viruses in our region:


This virus enters our body through breathing in infected droplets or touching your face after touching contaminated materials.

Adenoviruses most commonly cause respiratory illness, but may also cause gastroenteritis, conjunctivitis, cystitis (bladder infection) and rashes.

Young infants and immunocompromised patients are more susceptible to the severe complications of adenovirus infection.

There is no cure or vaccination available for this virus.


Coronaviruses are a group of viruses that include SARS-CoV (severe acute respiratory syndrome coronavirus), MERS-CoV (Middle East respiratory syndrome coronavirus) and SARS-CoV-2, which causes Covid-19.

Coronaviruses are zoonotic, meaning that they originate in animals that spread it to humans.

Human-to-human transmission is through infected droplets spread via sneezing and coughing.

Common symptoms include fever, cough and shortness of breath.

Some people might have no symptoms, while others like the elderly, those with chronic diseases and the immunocompromised, might experience severe respiratory distress and pneumonia.

There is currently no cure or vaccine for a coronavirus infection.

Prevention of infection includes regular hand hygiene and avoiding those with symptoms like coughing and sneezing.

Dengue virus

This virus, spread through the bite of an infected Aedes aegypti mosquito, causes dengue fever, and its more serious variation, dengue haemorrhagic fever.

Symptoms include high fever, rashes, severe headache, pain behind the eyes, and muscle and joint pain, which can be very severe.

There is no cure for dengue fever; treatment is usually supportive, like rest and taking plenty of fluids.

Those who experience severe bleeding might require blood transfusion.

Prevention usually consists of eliminating mosquito-breeding sites and fogging in areas where dengue has occurred.

A vaccine is available for this disease.

Some of the viruses on this list are preventable through vaccination.Some of the viruses on this list are preventable through vaccination.

Hepatitis B virus

This virus is transmitted when body fluids, like blood and semen, from an infected person enter the body of another person.

For example, infected mothers can pass the virus to their newborns during birth, an infected person can pass it to their sexual partner and an infected drug addict can pass the virus by sharing needles with another person.

The hepatitis B virus causes acute viral hepatitis (liver infection), which starts with general ill health, loss of appetite, nausea, vomiting, body aches, mild fever and dark urine, then progresses to the development of jaundice.

Complications include fulminant hepatic or liver failure, chronic hepatitis, cirrhosis and hepatocellular carcinoma (liver cancer).

Hepatitis B is treated with antiviral drugs and is preventable with vaccination.

Herpes viruses

There are eight types of herpes viruses, out of over 100, that routinely affect humans: herpes simplex virus type 1 (HSV-1) and 2 (HSV-2), varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, and human herpesvirus 6,7 and 8.

We are probably most familiar with the first two types of herpes virus.

HSV-1 is a highly contagious and common infection throughout the world, passing through contact with the saliva, cold sores or mouth surface of an infected person.

Most infections occur in childhood and are lifelong, but usually without symptoms.

This virus commonly causes oral herpes, of which one symptom is cold sores or ulcers in or around the mouth.

It can also cause genital herpes when an infected person performs oral sex on another person, with one symptom being cold sores or ulcers on the genitals.

HSV-2 is a sexually-transmitted infection (STI) that can result in genital herpes.

Although it is also a lifelong infection, many infected people experience either mild or no symptoms.

Antiviral drugs can suppress the virus and reduce the severity and length of the symptoms, but cannot remove the virus from your body.

Human immunodeficiency virus (HIV)

HIV is transmitted by sexual contact, blood transfusion, contaminated needles and during delivery of a newborn.

It attacks our immune cells (CD4+) and lowers its count progressively, resulting in the patient becoming immunocompromised, or too weak to fight off normal infections.

This condition is known as Acquired Immune Deficiency Syndrome (AIDS).

Patients can prolong their lives by taking anti-retroviral therapy, which is a combination of HIV drugs, but there is no cure for HIV/AIDS, nor is there a vaccine for it.

Practising safe sex, screening for the virus during blood donation and avoiding needle-sharing among drug addicts are among the ways to prevent HIV infection.

Human papillomavirus (HPV)

This virus is transmitted by direct contact and is the most common STI worldwide.

Most HPV infections cause no symptoms and resolve spontaneously.

In some, the infection persists and results in warts or pre-cancerous lesions that can lead to cancer of the cervix, vulva, vagina, mouth or throat.

There is no treatment for an HPV infection, but there are vaccines to prevent infections by the most common types of this virus.

Measles virus

This virus is transmitted through contact with droplets from an infected person when they cough or sneeze.

It causes measles, which is a very contagious infection.

Initial symptoms typically include fever, cough, runny nose and inflamed eyes.

Small white spots known as Koplik’s spots may form inside the mouth.

A red, flat rash usually starts on the face, then spreads to the rest of the body.

There is no cure for this disease; treatment is supportive.

Measles is preventable through vaccination.

Mumps virus

This virus is transmitted through contact with droplets from an infected person when they cough or sneeze.

It causes mumps. Symptoms include swollen and tender salivary or parotid glands, difficulty chewing, fever, headache, muscle aches and loss of appetite.

There is no cure for this disease; treatment is supportive.

Warm saltwater gargles, soft foods and extra fluids may also help relieve symptoms.

Mumps is preventable through vaccination.


This virus usually causes infections in animals, but one type, parvovirus B19, only infects humans.

Parvovirus B19 is highly infectious and spreads through contact with droplets from an infected person when they cough or sneeze.

It can infect children and causes the classic “slapped-cheek” rash of erythema infectiosum.

There is no cure nor vaccine for parvovirus B19; treatment is supportive.


This virus is transmitted via the faecal-oral route, meaning that the virus enters the body through the ingestion of infected faecal matter.

This often occurs through contaminated food and water.

The virus causes poliomyelitis, commonly called polio, which mainly affects young children.

Initial symptoms of polio include fever, fatigue, headache, vomiting, stiffness in the neck and pain in the limbs.

In a small proportion of cases, the disease causes paralysis, which is often permanent.

There is no cure for this condition.

Supportive treatments such as bed rest, pain control, good nutrition and physical therapy to prevent deformities from occurring over time, can help reduce the long-term symptoms due to muscle loss.

Polio is preventable through vaccination.`

Rabies virus

This virus usually infects domestic and wild animals, and is spread to humans through saliva when an infected animal bites them.

Over 90% of human cases worldwide are caused by dogs.

Initially, non-specific symptoms such as fever, sore throat, malaise, headache, nausea and vomiting will occur.

There may be discomfort or a prickling or itching sensation at the site of bite, progressing within days to symptoms of cerebral dysfunction, anxiety, confusion and agitation.

As the disease progresses, the person may experience delirium, abnormal behaviour, hallucinations and insomnia.

An irrational fear of water (hydrophobia) and of fresh air (aerophobia) are unique signs of rabies infection in humans.

Antiviral treatment is not effective, but it is possible to prevent rabies by quickly treating the wound and administering human rabies immunoglobulin, as well as the rabies vaccine.

A pet dog gets vaccinated against rabies in Kuching in this 2017 filepic, when the rabies outbreak in the state of Sarawak first began. The rabies virus can pass from an infected animal to a human through its saliva, usually via a bite.A pet dog gets vaccinated against rabies in Kuching in this 2017 filepic, when the rabies outbreak in the state of Sarawak first began. The rabies virus can pass from an infected animal to a human through its saliva, usually via a bite.


This virus is transmitted via the faecal-oral route.

It causes viral gastroenteritis, with acute onset of vomiting and diarrhoea that lasts between four to seven days.

It occurs most often in children under the age of two years.

There is no cure for this disease; treatment is supportive and consists of fluid and salt replacement either orally or intravenously.

Rotavirus infection is preventable through vaccination.

Rubella virus

This virus is transmitted through contact with droplets from an infected person when they cough or sneeze.

It causes rubella, also known as German measles or three-day measles.

A rash may start on the face and spread to the rest of the body. Fever, sore throat and fatigue may also occur.

There is no cure for this disease; treatment is supportive.

Rubella is preventable through vaccination.

Associate Professor Dr Mohammad Nazmul Hasan Maziz is a lecturer in microbiology at the Perdana University Graduate School of Medicine. This article is courtesy of Perdana University. For more information, email starhealth@thestar.com.my. The information provided is for educational and communication purposes only and it should not be construed as personal medical advice. Information published in this article is not intended to replace, supplant or augment a consultation with a health professional regarding the reader’s own medical care. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.

Article type: metered
User Type: anonymous web
User Status:
Campaign ID: 18
Cxense type: free
User access status: 3

Virus , infectious diseases


Did you find this article insightful?


90% readers found this article insightful

Across the site