What Are Bacteria?
Bacteria are tiny single-celled organisms that can reproduce on their own by splitting in two through a process called binary fission. They come in different shapes — some are round like balls, some are rod-shaped like tiny cylinders, and some are spiral-shaped like corkscrews. The human body contains about 38 trillion bacteria, most of them living in your gut where they help you digest food and produce important vitamins. Many bacteria are not only harmless but actually beneficial — bacteria are used to make yogurt, cheese, and some medicines. Only a small percentage of bacteria cause disease in humans.
What Are Viruses?
Viruses are tiny packages of genetic material — either DNA or RNA — surrounded by a protein coat called a capsid. They are about 100 times smaller than bacteria, which means you would need a powerful electron microscope to see most of them. Unlike bacteria, viruses cannot reproduce on their own. Instead, they invade a host cell and hijack the cell’s machinery to make copies of themselves, often destroying the cell in the process. Once the cell bursts open, the new virus copies spread out to infect more cells.
Helpful vs. Harmful Bacteria
Not all bacteria are bad for you — in fact, most of the bacteria in and on your body are helpful. Your gut bacteria, known as your microbiome, help break down food, produce vitamins like vitamin K and some B vitamins, and protect you from harmful germs. Bacteria in the soil break down dead plants and animals, recycling nutrients back into the earth. In the food industry, bacteria are essential for making fermented foods like yogurt, cheese, sauerkraut, and sourdough bread. However, harmful bacteria can cause diseases like strep throat, urinary tract infections, Lyme disease, and tuberculosis.
Common Diseases Caused by Viruses
Viruses are responsible for many of the illnesses you have probably experienced or heard about. The common cold is caused by more than 200 different viruses, which is why you can catch a cold over and over again. Influenza, or the flu, is caused by influenza viruses that change slightly every year, which is why a new flu vaccine is needed each season. Other viral diseases include COVID-19, chickenpox, measles, and stomach bugs caused by norovirus. Vaccines have been developed to prevent many serious viral diseases, saving millions of lives every year.
Antibiotics and Why They Only Work on Bacteria
Antibiotics are medicines that kill bacteria or stop them from growing, but they have absolutely no effect on viruses. This is because antibiotics target specific structures in bacterial cells — like their cell walls — that viruses simply do not have. Taking antibiotics for a viral illness like a cold or the flu will not help you get better and can actually cause harm. Overusing antibiotics leads to antibiotic resistance, where bacteria evolve to survive the medication, creating dangerous “superbugs” that are much harder to treat. This is why doctors carefully determine whether an infection is bacterial or viral before prescribing antibiotics.
How Your Immune System Fights Back
Your body has a powerful defense system that fights both bacteria and viruses. Your skin and mucous membranes act as physical barriers, keeping most germs out. White blood cells patrol your body and attack invaders they detect. When you get a fever, your body is actually raising its temperature to create an environment that makes it harder for germs to survive. Your immune system also creates antibodies — special proteins that recognize and target specific germs — so that if you encounter the same germ again, your body can fight it off much faster.
Vaccines: Training Your Immune System
Vaccines are one of the greatest medical inventions in history. They work by introducing a harmless version of a virus or bacteria — or just a piece of one — into your body so your immune system can learn to recognize it. When your immune system encounters the real germ later, it already knows how to fight it and can respond quickly before you get seriously ill. Vaccines have eliminated smallpox, nearly wiped out polio, and dramatically reduced deaths from measles, mumps, and many other diseases. Getting vaccinated protects not only you but also people around you who may not be able to get vaccinated themselves.
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