What’s the difference between bacteria and viruses?
Bacteria
Bacteria are single-celled organisms that have the ability to reproduce, and feed themselves. Bacteria are found everywhere, including the air, water, and soil. Most bacteria are beneficial for our health and the health of earth’s ecosystems, with only 1% of bacteria causing diseases.
They divide and multiply very quickly. One cell can become 1 million cells in a few hours. Bacteria are about 1000 nm in size, which means they are visible under a light microscope.
Bacteria can be treated with antibiotics. Some bacterial infections get better with no antibiotics. Your immune system deals with them and you get better.
Examples of bacteria (and the infection they cause)
Bordetella pertussis. This is the cause of whooping cough (also known as ‘pertussis’), and is a very contagious respiratory illness.
Clostridium difficile (‘C. diff’) – is a type of bacteria in your intestines that’s normally harmless. However, using antibiotics or having an weakened immune system can cause overgrowth of these bacteria. This leads to a gastrointestinal infection with an inflamed colon and diarrhoea.
Corynebacterium diphtheriae. Diphtheria is an infection caused by this bacterium
Escherichia coli (‘E. coli’). This is the commonest cause of UTIs.
Heliobacter pylori (‘H. pylori’) – causes stomach ulcers and chronic gastritis (inflammation of the stomach).
Legionella pneumophila. This is the cause of Legionnaires’ Disease (often a pneumonia). The most common form of transmission is from contaminated water; especially related to air conditioning and water systems (e.g. swimming pools and spas).
Methicillin-resistant staphylococcus aureus (MRSA) – is a type of bacteria that’s become resistant to antibiotics. It can be dangerous, especially for people who have compromised immune systems.
Mycobacterium tuberculosis. Tuberculosis is a contagious disease caused by the Mycobacterium tuberculosis bacteria. It usually leads to long term lung infection.
Neisseria meningitidis. This is often referred to as ‘meningococcus’, is a bacterium that can cause meningitis.
Neisseria gonorrhoeae – causes gonorrhoea, which is a sexually transmitted disease (STD).
Salmonellae – are causes of gastroenteritis (that can be ‘food poisoning’). They live in the gastrointestinal tracts of humans and other animals. Symptoms include abdominal pain, diarrhoea and vomiting.
Staphylococcus aureus (or ‘staph’) – is a type of bacteria found on human skin, in the nose, armpit and groin. They usually don’t cause harm, but can make you sick under the right circumstances. Foe examples, S. aureus is the leading cause of skin and soft tissue infections, such as abscesses and cellulitis.
Cellulitis
It can also cause more serious infections, such as pneumonia, endocarditis (infection of the heart valves), and bone and joint infections.
Streptococcus pneumoniae. Bacterial pneumonia is a lung infection that can be caused by different kinds of bacteria, including Streptococci. Other bacteriae include Haemophilus influenzae, Klebsiella pneumoniae, or Pseudomonas aeruginosa.
Streptococcus pyogenes (or ‘Strep A’). This is the bacterium that causes ‘strep throat’. It can cause other infections including scarlet fever, impetigo and rheumatic fever.
Treponema pallidum. Syphilis is a sexually transmitted disease (STD) caused by this bacterium.
There is more information about bacteria on this US website here.
Viruses
Viruses are present in animals, plants, and other living organisms – and many cause diseases. They are microorganisms that are smaller than bacteria; usually about 20-400 nm, and can be visualised by an electron microscope.
Viruses cannot grow or reproduce without the help of a separate living cell. This is because although viruses have genes, they do not have a cellular structure to perform their own metabolism, thus requiring a host cell to make new products. Therefore viruses attach themselves inside healthy cells once they get inside your body; and use the cell’s nucleus to reproduce themselves.
Antibiotics have no effect on viruses. Vaccines can prevent you getting viruses. Antiviral drugs can be used to treat some viruses but have no effect on bacteria.
Examples of viruses (and infections they cause)
Epstein-Barr virus (EBV). Causes infectious mononucleosis (or ‘glandular fever’ or ‘kissing disease’). The main symptoms include extreme tiredness, swollen glands in the neck and a high temperature. Glandular fever usually gets better by itself.
Hepatitis A,B, C, D and E. Viral hepatitis – is an infection that causes liver inflammation and damage.
Herpes simplex virus (HSV). Causes ‘cold sores’ and encephalitis. The latter is a rare but serious condition.
Human immunodeficiency virus (HIV). Acquired immunodeficiency syndrome (AIDS) is a long-term condition caused by this virus.
Human papillomavirus (HPV). This is a common sexually transmitted disease (STD) and can cause certain cancers (e.g. cervical), and genital warts.
Influenza viruses. One of causes of influenza (‘flu’). The symptoms of flu are similar to a very bad cold. They include a high temperature, an aching body, tiredness and a headache.
Measles morbillivirus (MeV; also called measles virus, MV) – causes measles. The main symptom is a red-brown blotchy rash. It usually gets better of its own accord. But it can be serious in some people.
Paramyxovirus. This causes mumps, an infection that causes painful swellings in the sides of your face, and a tender swollen jaw. It usually gets better on its own, but can make some people very ill.
Poliovirus. This virus can poliomyelitis, which is an infection of the spinal cord, causing inability to move parts of the body (paralysis), and in some cases can become life threatening.
Rabies lyssavirus. This causes rabies, a rare but very serious infection of the brain and nerves. It’s usually caught from the bite or scratch of an infected animal, most often a dog.
Respiratory syncytial virus (RSV). For most adults and healthy children, RSV infection causes symptoms similar to those of a cold.
Rhinoviruses. They are the most common viruses in humans, and the commonest cause of the common cold.
Severe acute respiratory syndrome coronavirus 2 (or SARS-CoV-2). This is the cause of COVID-19.
Varicella zoster virus (VZV). Chickenpox (also known as ‘varicella’) and shingles are highly contagious diseases caused by VSV.
Variola virus. This virus is the cause of smallpox, which has now been eradicated. Before this, it was a serious infectious disease. People who had smallpox had a fever and a distinctive, progressive skin rash.
Summary
We have described what’s the difference between bacteria and viruses. We hope it has been helpful.
Other resources More information on viruses on this US website here.
