Infection occurs when harmful germs (microorganisms) enter the body and multiply. Different kinds of germs cause different infections, but all infection involves the entry of bacteria, viruses, fungi or parasites and their subsequent multiplication and damage to the host organism.
There are five major types of infectious agents: bacteria, viruses, fungi, protozoa and helminths. Bacteria are ancient organisms that have evolved over time to adhere to cells, produce paralyzing poisons and other toxins, suppress or evade our immune system’s antibodies and resist drugs and antibiotics. They are responsible for a variety of infections including strep throat, tuberculosis, staph skin infections and urinary tract and bloodstream infections.
Viruses are simple packets of nucleic acid, either DNA or RNA, surrounded by protein shells and sometimes fatty materials called lipids. Outside a living cell, a virus lacks the raw materials needed to reproduce. However, once a virus enters a living cell, it hijacks the cell’s metabolic machinery to reproduce itself. Once a virus has multiplied, it can break out of the cell and infect other cells.
Viruses, fungi and parasites can spread from person to person in many ways including direct contact, airborne transmission or vector-borne transmission (by ticks or mosquitoes). Direct contact transmission includes infections of the skin, such as scabies, head lice and impetigo; respiratory infections such as influenza, whooping cough and pneumonia; and gastrointestinal infections like diarrhoea and gastroenteritis. Airborne transmission happens when a person with an infectious disease coughs, sneezes or sings and contaminates surfaces such as hands, cups, toys or food.