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Infectious Diseases
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Infections

Infectious Diseases

Infectious diseases are illnesses caused by harmful germs, including bacteria, viruses, fungi, and parasites, that enter the body.

What are infectious diseases?

Germs, or microbes, are found everywhere—in the air, soil, and water. There are also germs on your skin and in your body. Many of them are harmless, and some can even be helpful, but some can make you sick. Infectious diseases are diseases that are caused by germs.

What are the different types of germs that cause infectious diseases?

There are four main types of germs:

  • Bacteria are one-celled germs that multiply quickly. They may give off toxins, which are harmful chemicals that can make you sick. Strep throat and urinary tract infections are common bacterial infections.
  • Viruses are tiny capsules that contain genetic material. They invade your cells so that they can multiply, which can kill, damage, or change the cells and make you sick. Viral infections include HIV and the common cold.
  • Fungi are primitive plant-like organisms such as mushrooms, mold, mildew, and yeasts. Athlete's foot and yeast infections are common fungal infections.
  • Parasites are animals or plants that survive by living on or in other living things. Malaria and head lice are infections caused by parasites.

How can you get infectious diseases?

There are many ways to contract an infectious disease, depending on the type of germ:

  • Through direct contact with a person who is sick, including kissing, touching, sneezing, coughing, and sexual contact. If you are pregnant, you can pass some germs to your fetus.
  • Through indirect contact, when you touch something that has germs on it, such as a door handle touched by someone who is sick.
  • Through insect or animal bites.
  • Through contaminated food, water, soil, or plants.

What are the symptoms of infectious diseases?

Infectious diseases can cause many different symptoms. Some are so mild that you may not even notice them, while others can be life-threatening.

What are the treatments for infectious diseases?

There are treatments for some infectious diseases, such as antibiotic, antiviral, antifungal, and anti-parasitic medicines. For other infections, such as some caused by viruses, you can only treat your symptoms while you wait for your immune system to fight off the germ.

Can infectious diseases be prevented?

You can take steps to help prevent many infectious diseases:

  • Get vaccinated.
  • Wash your hands often.
  • Pay attention to food safety.
  • Avoid contact with wild animals.
  • Use a latex condom every time you have sex (or polyurethane if you or your partner is allergic to latex).
  • Don't share items such as toothbrushes, combs, and straws.

Frequently Asked Questions

Seek immediate emergency care if you experience difficulty breathing, chest pain, confusion, severe dehydration (such as inability to keep fluids down), or a high fever that does not respond to medication. For pregnant individuals, any fever or unusual symptoms should be evaluated urgently to protect both the mother and the fetus. If you have a weakened immune system, even mild symptoms of an infection can become severe quickly and require prompt medical attention.
Vaccines train your immune system to recognize and fight specific germs without causing the disease. They introduce a harmless piece of the germ or a weakened version, prompting your body to produce antibodies. This creates immunity so that if you are exposed to the real germ later, your body can destroy it quickly. Vaccination schedules vary by age, health status, and travel plans, so consult your healthcare provider to determine which vaccines are recommended for you, including routine childhood vaccines and adult boosters like Tdap or flu shots.
Practice rigorous hand hygiene by washing hands with soap and water for at least 20 seconds, especially after touching shared surfaces like door handles, keyboards, or light switches. Avoid touching your face, particularly your eyes, nose, and mouth, with unwashed hands. If you are sick, stay home to prevent direct contact transmission. Additionally, regularly disinfect high-touch surfaces in your workspace with an EPA-registered disinfectant to reduce indirect contact risks.
While only a healthcare provider can definitively diagnose the cause, viral infections often present with a gradual onset of symptoms like body aches, fatigue, and a runny nose, and typically resolve on their own within a week. Bacterial infections may start more suddenly and can include higher fevers, localized pain (such as in the throat or ears), and symptoms that persist or worsen after several days. If symptoms last longer than 10 days or improve and then suddenly get worse, see a doctor, as this may indicate a secondary bacterial infection.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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