Microbes, Germs and Antibiotics

Authored by , Reviewed by Dr Hannah Gronow | Last edited | Meets Patient’s editorial guidelines

Added to Saved items

This leaflet gives a brief overview of the different types of germs (microbes) that can cause infections and a very brief overview of the use of antibiotic medication.

Healthcare professionals classify 'germs' (microbes) into different groups. The most common groups of microbes that cause illness are described below.


There are many different types of bacteria. Some are helpful and protective to humans. Some flourish naturally in our bodies - particularly in the bowel and vagina - and help to protect the body from infections. However, infections with certain bacteria can cause serious illnesses such as meningitis, pneumonia, tuberculosis, etc. A bacterial infection may be treated with a course of antibiotic medication.


These are smaller and different to bacteria. Many different types exist. Most of the common 'minor' illnesses are caused by viruses. For example, colds, coughs, sore throats, chickenpox and some other rashes. Most common infections in the community are due to a viral infection.

Viral infections are much more common than bacterial and fungal infections.

For many viral infections there are no effective antiviral medicines (unlike antibiotics for bacteria). Fortunately, the immune system in the body usually fights off most viral infections within a few days. Taking 'symptomatic' treatments for a high temperature (fever) or catarrh, such as paracetamol and/or ibuprofen, resting and drinking plenty are usually all that needs to be done to get better.

There are some antiviral medicines that are used for certain infections - such as antiretroviral medicines used to treat HIV. Another example is aciclovir and related medicines which are used to treat certain herpes virus infections. As a rule, antiviral medicines do not clear the virus from the body. They usually work by stopping the virus from multiplying and so 'control' the virus and the infection that it causes.


Many types of fungi exist and cause problems in humans, animals and plants. Fungal infections commonly affect the skin and nails in humans. They can cause ringworm, athlete's foot, other localised skin rashes and infections in and around nails. Modern creams usually work well to clear a local fungal rash quickly. However, nail infections can be rather stubborn and may need long-term treatment of antifungal medicines taken by mouth.

Most fungi are free-living in the environment and few of these are capable of causing infection in an otherwise healthy person. However, they can cause serious infections in patients with weakened immune systems (for example, those who have recently received chemotherapy for cancer).


Yeasts are actually a type of fungus. There are different yeasts which cause various infections. The most common yeast infection is thrush. This is due to a yeast called candida which thrives in moist, airless, warm areas of the body. It can cause infections in the vagina and infections in the mouth. It can cause nappy rash in babies and it can also sometimes infect other areas of the body. Treatment of yeast infections usually works well with anti-yeast creams and medicines.


A parasite is a type of germ that needs to live on or in another living being (host) to survive. It gets its food from its host.

Parasites are usually found in contaminated water or food. They can also get into the body by insect bites or by sexual contact. Parasitic infections are more common in the tropics and subtropics. They can occur in the UK but the serious infections are more typically seen in people who have weakened immune systems (for example, those with HIV or those people taking chemotherapy for types of cancer).

Examples of diseases caused by parasites are malaria, amoebic dysentery and giardia. Threadworms, hookworms and tapeworms are also parasites. 

Antibiotics are not a  'cure all' for infections. Antibiotics will only clear infections caused by germs such as bacteria and some parasites. They do not work when an infection is caused by viruses, fungi or yeasts. As mentioned, most common infections are caused by viruses when an antibiotic will not be of use. Even if you have a bacterial infection, the immune system can clear most bacterial infections. This means that antibiotics are not needed for minor infections (for example, an ear or throat infection in an otherwise fit person).

However, you do need antibiotics if you have certain serious infections caused by bacteria, such as meningitis, pneumonia or kidney infections. In these situations, antibiotics are often life-saving. When you are ill, doctors are skilled at checking you over to rule out serious illness and to advise if an antibiotic is needed.

Some possible problems with antibiotics

Antibiotics are not without problems. This is why it is not usually good practice to take antibiotics 'just in case' an infection is bacterial, but to take them only when they are really needed. For example:

  • Antibiotics can cause side-effects such as allergies, diarrhoea, rashes and nausea. Side-effects are quite common. Most side-effects are not serious, but some people have died from a severe allergic reaction to an antibiotic.
  • Antibiotics can kill off normal 'defence' bacteria which live in the bowel and vagina. This may then allow other infections - for example, thrush - to develop.
  • Overuse of antibiotics has led to some bacteria mutating and becoming resistant to some antibiotics which may then not work when really needed. For example, meticillin-resistant Staphylococcus aureus (MRSA) is a bacterium that has become resistant to many different antibiotics and is often very difficult to treat. Another example is some bacteria produce chemicals called enzymes such as extended-spectrum beta-lactamases (ESBLs), which make them resistant to some antibiotics. The more antibiotics are used, the greater the problem of antibiotic resistance.
  • When antibiotics are appropriately prescribed, it is important to take them as directed on your prescription and to complete the full course of treatment. That often means continuing to take the antibiotics as directed when you feel better. Failing to finish a course of treatment or failing to take the antibiotics as prescribed, can actually result in the growth of resistant antibiotics.
  • Some antibiotics may interact with other medicines that you might take. This may cause reactions, or reduce the effectiveness of one or other of the treatments.
  • Food and drink affect the absorption of some antibiotics - so follow the instructions on how to take a course of antibiotics.

See the separate leaflet called Antibiotics for more information.

In summary

Doctors are skilled in diagnosing which conditions are in need of antibiotics. So do not be surprised if a doctor does not recommend an antibiotic for conditions caused by viruses or non-bacterial infections, or even for a mild bacterial infection.

Most simple coughs, colds, sore throats and influenza are caused by viruses and an antibiotic will not work.

Occasionally, a viral infection or minor bacterial infection develops into a more serious 'secondary' bacterial infection. You should see a doctor to review the situation if an illness appears to change, becomes worse, does not go after a few days or if you are worried about any new symptom that develops.



Are you protected against flu?

See if you are eligible for a free NHS flu jab today.

Check now

Further reading and references