What to Know About How Diseases Spread

Medically Reviewed by Sabrina Felson, MD on March 21, 2023
4 min read

Diseases live and spread in humans, animals, and non-living objects. Contagious diseases can infect people from skin-to-skin contact, breathing, eating, and other pathways.

Most contagious diseases are one of four types.

Bacterial infections. Bacteria are tiny living things made of one cell. They’re too small to see with the naked eye. Most types of bacteria are harmless or helpful, like gut bacteria. Other types of bacteria can cause mild to life-threatening illnesses.

Viral infections. Viruses are tiny pieces of genetic code that can only reproduce (or replicate) using a host organism. Living things like animals are made of tiny cells that multiply. New cells replace old ones to help the living thing grow and stay healthy. 

The living thing’s genetic code tells its cells how to copy themselves. Viruses use their genetic code to trick the cell into making copies of the virus instead. This can damage the cells and make you sick.

Parasitic infections. Parasites are living things that feed off of your body. Unlike bacteria or viruses, they can’t survive on their own without a living host.

Parasites can be bugs like lice and tapeworms, tiny single-celled living things like protozoa, or other types.

Fungal infections. Fungi are a type of living thing that includes mushrooms, mold, and yeast. Some types of fungi can make you sick. 

Fungi such as yeast infections and athlete’s foot cause infections on your skin and other body surfaces. Other types of fungi, like black mold, can cause breathing problems and sinus infections.

Between humans, diseases can spread via:

Skin-to-skin contact. Many diseases spread through touching other peoples’ skin. Your skin naturally sheds many dead cells throughout the day. Someone with an infection may shed skin cells containing germs. 

Those infected cells can touch broken skin or mucous membranes like your mouth. This can cause infection.

Sexual activity. Diseases like AIDS and syphilis spread through semen, vaginal fluids, and saliva. Sexual activity like kissing, oral sex, and penetration can spread germs. 

Some germs spread most often through sexual contact. This category of diseases is called sexually transmitted infections

Childbirth. Someone who is pregnant can pass a disease onto their child. This is called congenital transmission. Congenital disease transmission can happen even if the parent doesn’t know they have the disease.

Germs can spread to a baby through food that the parent eats at the time of childbirth, or from breastfeeding.

The air. The air you breathe can carry droplets containing particles of a disease. An infected person spreads those tiny droplets by sneezing, coughing, or talking. The droplets can enter through your nose, mouth or eyes when you inhale.

Diseases often live and spread in animals and bugs. Some animal infections can also infect humans. Sicknesses that animals pass to humans are called zoonotic diseases.

Bites from infected animals. Some diseases are spread directly from animals to humans through bites or scratches. Others can be spread to humans by vectors, such as insects, that carry the infection from an animal to a human. These vectors include:

  • Ticks
  • Mosquitoes
  • Fleas
  • Flies

Some bacteria, viruses, and other germs may also enter through the broken skin.

Contact with animals. Animals can spread diseases in other ways besides bites. Germs can spread through animals’ body fluids such as:

  • Feces
  • Saliva
  • Blood
  • Urine
  • Vomit

Touching or petting animals can also spread disease. Fleas and germs may live in the fur they shed.

Eating animals. Cooking meat kills germs through heat. Bacteria live inside meat that’s not cooked thoroughly. Eating raw or undercooked meat and eggs can cause infections from these bacteria.

Many germs can survive on or inside non-living objects. Touching these objects and then touching your mouth, nose or eyes can make you sick. Some common items that spread disease include:

  • Fruits and vegetables
  • Water
  • Kitchen utensils
  • Medical tools
  • High-touch surfaces like doorknobs
  • Body fluids from people or animals
  • Clothing
  • Bedsheets

You can take easy steps to prevent catching and spreading contagious diseases.

Safe sex. Use condoms during sex to help prevent sexually transmitted diseases. You can get tested for common sexually transmitted illnesses (STIs). 

Get vaccinated. Vaccines help your immune system build up special immune cells to fight off diseases. They can prevent severe illness if you’re exposed to germs. They can also make it less likely for you to spread germs to others.

Keep your hands clean. Soap and hand sanitizer can get rid of many germs that cause disease. Washing your hands can help prevent sickness if you touch your eyes, nose, or mouth.

Cover your face while coughing and sneezing. Coughing and sneezing send high amounts of germs into the air. Covering your mouth and nose with the inside of your elbow reduces the amount of germs other people might breathe in.