Infectious Diseases

Infectious diseases are illnesses caused by harmful organisms (pathogens) that get into your body from the outside. Pathogens that cause infectious diseases are viruses, bacteria, fungi, parasites and, rarely, prions. You can get infectious diseases from other people, bug bites and contaminated food, water or soil.

What’s the difference between infectious diseases and noninfectious diseases?

Infectious diseases are caused by harmful organisms that get into your body from the outside, like viruses and bacteria. Noninfectious diseases aren’t caused by outside organisms, but by genetics, anatomical differences, getting older and the environment you live in. You can’t get noninfectious diseases from other people, by getting a bug bite or from your food.

The flu, measles, HIV, strep throat, COVID-19 and salmonella are all examples of infectious diseases. Cancer, diabetes, congestive heart failure and Alzheimer’s disease are all examples of noninfectious diseases.

What are the types of infectious diseases?

Infectious diseases can be viral, bacterial, parasitic or fungal infections. There’s also a rare group of infectious diseases known as transmissible spongiform encephalopathies (TSEs).

• Viral infections. Viruses are a piece of information (DNA or RNA) inside of a protective shell (capsid). Viruses are much smaller than your cells and have no way to reproduce on their own. They get inside your cells and use your cells’ machinery to make copies of themselves.
• Bacterial infections. Bacteria are single-celled organisms with their instructions written on a small piece of DNA. Bacteria are all around us, including inside of our body and on our skin. Many bacteria are harmless or even helpful, but certain bacteria release toxins that can make you sick.
• Fungal infections. Like bacteria, there are many different fungi. They live on and in your body. When your fungi get overgrown or when harmful fungi get into your body through your mouth, your nose or a cut in your skin, you can get sick.
• Parasitic infections. Parasites use the bodies of other organisms to live and reproduce. Parasites include worms (helminths) and some single-celled organisms (protozoa).
• Transmissible spongiform encephalopathies (TSEs/prion diseases). TSEs are caused by prions — faulty proteins that cause other proteins in your body, usually in your brain, to become faulty as well. Your body is unable to use these proteins or get rid of them, so they build up and make you sick. Prions are an extremely rare cause of infectious diseases.

What are common infectious diseases?

Infectious diseases are extremely common worldwide, but some are more common than others. For instance, each year in the United States, 1 out of every 5 people is infected with the influenza virus, but less than 300 people are diagnosed with prion diseases.

Some of the most common infectious diseases are listed here by type.

Common infectious diseases caused by viruses:

• Common cold.
• The flu (influenza).
• COVID-19.
• Stomach flu (gastroenteritis).
• Hepatitis.
• Respiratory syncytial virus (RSV).

Common infectious diseases caused by bacteria:

• Strep throat.
• Salmonella.
• Tuberculosis.
• Whooping cough (pertussis).
• Chlamydia, gonorrhea and other sexually transmitted infections (STIs).
• Urinary tract infections (UTIs).
• E. coli.
• Clostridioides difficile (C. diff).

Common infectious diseases caused by fungi:

• Ringworm (like athlete’s foot).
• Fungal nail infections.
• Vaginal candidiasis (vaginal yeast infection).
• Thrush.

Common infectious diseases caused by parasites:

• Giardiasis.
• Toxoplasmosis.
• Hookworms.
• Pinworms.

Who is most at risk for getting infectious diseases?

Anyone can get an infectious disease. You may be at an increased risk if your immune system is weakened or if you travel to areas with certain highly transmissible diseases.

People at higher risk of infectious disease include:

• Those with suppressed or compromised immune systems, such as those receiving cancer treatments, living with HIV or on certain medicines.
• Young children, pregnant people and adults over 60.
• Those who are unvaccinated against common infectious diseases.
• Healthcare workers.
• People traveling to areas where they may be exposed to mosquitoes that carry pathogens such as malaria, dengue virus and Zika viruses.

What complications are associated with infectious diseases?

Many infectious diseases resolve without complications, but some can cause lasting damage.

Serious and life-threatening complications of various infectious diseases include:

• Dehydration: Fever, vomiting, diarrhea.
• Pneumonia: Respiratory illness (viral or bacterial).
• Sepsis: Bacterial infections.
• Meningitis (swelling of the brain): Multiple causes, including bacterial, viral, fungal and parasitic infections.
• AIDS: HIV.
• Liver cancer: Hepatitis B, hepatitis C.
• Cervical cancer: Human papillomavirus (HPV).