Infection is caused by micro-organisms such as bacteria, viruses and fungi. These organisms are everywhere and most don’t cause disease, in fact some germs are useful (such as the bacteria in your gut that help you digest food). However, when these germs get into the wrong place in the body, they can lead to infection.
Infectious diseases spread from person to person in many ways, including direct contact (such as touching someone who has an infectious disease), sneezing and coughing into the air and then breathing it in, or sharing a toothbrush or drinking water with a sick person. They can also be spread when contaminated body fluids, such as urine, faeces, saliva, vomit and blood, come into contact with the eyes, nose or mouth. This includes head lice, scabies and many sexually transmitted infections like hepatitis B and human immunodeficiency virus (HIV).
Bacterial infections spread through direct contact with a person carrying them or indirectly when they touch surfaces such as taps, tables, door handles and toys that have the bacteria on them, or when they enter the body through broken skin (for example impetigo and boils). They can also be spread through droplets in the air, for instance when someone with a bacterial infection sneezes or coughs into the air. This is how people catch infections such as legionnaires’ disease, tuberculosis and whooping cough.
Viruses are even smaller than bacteria, consisting only of a protein coat and a core of genetic material (RNA or DNA). They can’t survive on their own so they must “infect” living cells to survive and reproduce. They are responsible for a wide range of infections, from the common cold and influenza to measles, chicken pox and genital herpes. Antiviral medicines such as aciclovir can treat most viral infections.