Infection is the presence of disease-causing organisms (pathogens) in body tissues. Many types of organisms can cause infections, including bacteria, viruses, fungi and parasites. These organisms can enter the body through openings such as the mouth, eyes or a cut in the skin, or by air or food contaminated with them. They may also enter the body through insect bites or by sexual contact, such as during unprotected intercourse or the handling of animal feces.
Bacteria are one-celled microorganisms that can cause infections such as strep throat, tuberculosis and urinary tract infections. These germs are ancient and have evolved many behaviors over 3 billion years, including the ability to attach themselves to cells, make paralyzing poisons and evade or suppress our immune systems.
Viruses are even smaller than bacteria, and they do not have the ability to reproduce on their own. They must infect a cell to survive and multiply. Once they enter a cell, they release their genetic material into the cell, which causes it to replicate itself and infect other cells around it. Once a viral infection takes hold, you may experience symptoms such as fever, aches and pains and tiredness.
Most infectious diseases are caused by bacteria, viruses and fungi. They are often spread by person to person (droplet transmission), or through contaminated objects such as cups, doorknobs and toys. They can also be spread by mosquitoes, lice or ticks that harbor and carry viruses or parasitic worms. Infections can also be caused by a number of changes in the world’s ecosystem, such as climate change, war and poverty, that create conditions where infectious diseases thrive.