An Overview of Infection

Types, Symptoms, Causes, Diagnosis, Treatment, and Prevention

Infection is a result of a microorganism (also called a pathogen) entering the body and causing harm. The organisms that cause infections are many and include things like viruses, bacteria, parasites, fungi, and prions.

There are many ways a person can contract on infection, including from another person, through contained foods or water, or a bug bite. Keep reading to learn about infections, including types, signs and symptoms, causes, and more. 

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Types of Infection

How infections spread through the body will depend on the type of microorganism causing it. Sometimes, a microorganism will overwhelm the immune system’s ability to fight a pathogen off. 

There are pathogens that have little effect on the immune system, while others produce inflammatory substances that trigger negative immune responses. There are also pathogens that might even be resistant to antibiotic therapy.

Viruses, bacteria, parasites, fungus, and prions are different types of pathogens that cause infections.

Viral Infections

Viral infections are caused by viruses. A virus will invade and attach itself to a cell. As the virus enters the cell, it leaves genetic material that forces the cell to replicate. When the cell dies, it will release new viruses that infect other cells.

Not all viruses destroy cells—some will change the function of a cell. Other viruses can lead to cancer because they force cells to uncontrollably replicate.

Viruses are known for causing a wide range of conditions, including the common cold and influenza. Other infectious diseases—like severe acute respiratory syndrome (SARS)—are also caused by viruses.

There are viruses that will stay with you for the rest of your life. These remain dormant within the body and then activate. Examples include herpes simplex virus or the varicella-zoster virus.

Treatment for a viral infection focuses on treating symptoms until the immune system clears out the infection. Some viral infections can be treated with antiviral drugs, including herpes and hepatitis C.

Bacterial Infections

A bacterial infection is caused by an excess of a harmful bacteria strain on or inside the body. Bacteria are small single-celled microorganisms. And they come in many different shapes and sizes and call be found in all types of environments, including in the human body. A bacterium can infect any part of the body.

There are bacteria in the body that can be helpful and not lead to disease. In fact, bacteria in the digestive tract can help with digestion and with keeping your body in balance.

Examples of bacterial infections include bacterial meningitis, pneumonia, tuberculosis, and food poisoning. 

Bacterial infections are usually treated with antibiotics. These medications affect bacterial growth—either they stop growth, or they kill existing bacteria. There are different types of antibiotics so what you are prescribed will depend on the source of the infection.

You should always take antibiotics as prescribed. Make sure you finish an entire course even if you find you are feeling better after a few days. Not finishing antibiotics can keep the infection from clearing out and could lead to antibiotic resistance.

Parasitic Infections

Parasites are organisms that need to feed off other organisms to survive. Some parasites do not noticeably affect people, while others will grow, replicate, and invade organ systems.

The term "parasite” includes protozoa (parasites consisting of only one cell) and worms (parasites that are larger and have many cells). Examples of parasitic infections are malaria, tapeworm infection, scabies, and pubic and head lice.

Parasitic infections are treated with specific drugs. This will differ based on the parasitic infection and how severe it is.

For example, malaria, which is a life-threatening condition that develops after an infected mosquito bites a person, is treated with anti-malarial drugs to kill the malaria parasite. Medications that treat malaria are chloroquine, Mepron (atovaquone), and doxycycline.

Fungal Infections

Fungi live in different types of environments. The most commonly known fungi include yeast, mold, and edible fungi (i.e., mushrooms). Much like bacteria, there are fungi that exist on the skin and within the body.

A type of fungus—called mycosis—can cause infection. Examples include fungal nail infections and athlete’s foot.

Fungal infections can also cause inflammation of the lungs caused by an aspergillus fungus. Fungal infections of the lungs can be life-threatening especially in people with weakened immune systems.

Other types of fungal infection can occur in the membranes of the mouth and reproductive organs. These can also be life-threatening.

While hard to believe, fungi can be beneficial to humans. For example, penicillin, a widely-used antibiotic is made from a type of fungi mold called Penicillium notatum. It is used to treat various bacterial infections.

Fungal infections are treated with antifungal medicines. The type of antifungal medicine your doctor will prescribe will depend on the type of fungal infection causing your symptoms.

Prion Diseases

A protein called a prion might be responsible for causing an infectious disease. Prion diseases include variably protease-sensitive prionopathy, fatal insomnia, and Creutzfeldt-Jakob disease (CJD). Fortunately, prion disease rare, without only about 300 cases reported in the United States.

Many prion conditions are inherited, but many others can be acquired. Such is the case with Creutzfeldt-Jakob disease. CJD can be inherited, but sporadic CJD will develop without risk factors. It can be acquired by exposure to infected tissue during a medical procedure.

Another type of CJD is related to mad cow disease. It can be transmitted when a person eats infected meat.

There are no cures for prion diseases. However, there are medicines to manage symptoms and slow down disease progression.

General Symptoms of Infection

The symptoms of infection will depend on the type of infection. General signs of infection might include:

Other more serious signs of infection that warrant calling your doctor include:

You should also notify your doctor if any infection symptoms worsen or don’t improve. Symptoms that recur or become chronic (lasting for a long time) should also be brought to your doctor's attention.

Complications

Most infections only cause minor problems. Some complications, like pneumonia or meningitis, can become life-threatening.

There are infections that have been linked to cancer. This includes human papillomavirus, which is linked to cervical cancer, Helicobacter pylori linked to stomach cancer, and hepatitis B and C linked to liver cancer.

Infections can also be silent and will appear much later in life. Chickenpox is one such example. It might result in shingles in older adults who had chickenpox when they were younger.

Causes and Risk Factors

There are many ways in which infections are acquired, including from direct and indirect contact, through contaminated food or water, or from a bug bite.

Direct Contact

An easy way to catch an infection is by coming in close contact with a person or animal with the infection:

  • Direct person-to-person contact can spread when a person who has an infection kisses, coughs, sneezes, or breathes near a person who isn’t infected.
  • Person-to-person infections can be spread through sexual contact.
  • A person who is pregnant can pass an infection to their unborn baby through the placenta or during labor and delivery.
  • A person who is breastfeeding can pass an infection through breast milk.
  • Animal to person contact can occur if you are bitten or scratched by an infected animal. Handling infected animal waste can be hazardous too.

Indirect Contact

Infections can be acquired through indirect contact. This is because many germs can linger on commonly touched surfaces, including doorknobs, faucets, and tables.

If you touch an object that has been handled by someone with an infection, you might pick up the germs left behind. If you then touch your eyes, nose, or mouth, you might become infected. That is why hand-washing is vital especially after touching commonly touched surfaces.

Indirect contact can be airborne through droplets from sneezing or coughing that can linger in the air for a short period. These droplets can make their way to a healthy person’s skin or can be inhaled into the respiratory passages and the lungs.

Contaminated Food or Water

You can become infected with an infectious disease through contaminated food or water. Escherichia coli (E.coli), for example, is a bacterium that can be found in foods like undercooked meat or unpasteurized milk.

Bug Bites

An infection can be acquired from an insect, such as a mosquito, flea, or tick. Mosquitoes can carry a malaria parasite or the West Nile virus. A deer tick carries bacteria that lead to Lyme disease.

Risk Factors

Anyone can catch an infection, regardless of the source. However, there are people who are more likely to get sick or have an increased risk for infection or infection complications.

People who are more susceptible to infections include:

  • Older adults
  • People who are pregnant
  • Young children
  • People with certain health conditions, including asthma, heart disease, and diabetes
  • People with autoimmune diseases who take immunosuppressive drugs and people with compromised immune systems from conditions like HIV/AIDs
  • People with some types of cancer

If you have an increased risk for infection or complications of infection, you should get in touch with your doctor if you experience symptoms of infection, regardless of the severity of those symptoms.

Diagnosis 

A number of infections can be distinguishable by the symptoms they cause. Your doctor might be able to diagnose you based on those symptoms, your medical history, and a physical exam.

In cases where it is harder to make a diagnosis, your doctor may request lab work. This might include taking samples of blood, urine, stool, sputum, or cerebrospinal fluid. These samples can help your doctor determine what is causing symptoms and the type of organism causing symptoms.

Your doctor might also request imaging, including an X-ray, computed tomography (CT) scan, or magnetic resonance imaging (MRI). Skin or tissue biopsies might be done to examine affected tissues.

Treatment       

Infections are treated according to their cause. The treatment for viral infections usually involves relieving symptoms until the immune system clears up the infection. However, some types of viral infections can be treated with anti-viral drugs.

Other medicines for treating infections are:

  • Antibiotics to treat bacterial infections
  • Anti-fungal drugs to treat fungal infections
  • Anti-protozoal drugs and other medicines to treat parasitical infections
  • Medicines to manage symptoms of infectious prion diseases

Talk to your doctor about the best course of treatment for infection symptoms and to help you to get healthy again. You should also let your doctor know if treatment doesn’t improve symptoms or makes them worse. 

Prevention 

There is no single way to prevent infection. Harvard Medical School has recommendations that can you stay healthy and potentially avoid an infection.

Ways you can prevent infection include:

  • Wash your hands often, especially before and after preparing food, going to the bathroom, and after other dirty tasks.
  • Clean surfaces often, especially those touched often, and disinfecting rooms that are known for having high bacteria content, i.e., the kitchen and bathroom.
  • Practice food-safety techniques, including rinsing foods with clean water before cooking or serving, and cooking meats thoroughly.
  • Stay up-to-date on vaccinations.
  • Take antibiotics only with a prescription and complete the recommended course.
  • Reduce your risk for sexually transmitted infections (STI) by using condoms or abstaining (not having sex), and getting regular STI check-ups.
  • Do not share personal care items such as toothbrushes, drinking glasses, kitchen utensils, etc.
  • Follow your doctor’s advice about traveling.
  • Prevent bug bite infections by using insect repellents and avoiding exposure as much as possible.
  • Make sure your pets are up-to-date on vaccinations, stay indoors, and see a vet regularly.

In addition to staying safe, eating a healthy diet and following an active lifestyle will help to keep your immune system strong enough to defend itself against infection. 

A Word From Verywell

Infections generally can be treated and managed at home. However, it is a good idea to contact your doctor if you have symptoms of infection that do not appear to be improving, are getting worse, or become chronic. Your doctor can prescribe additional medications to treat you or request additional testing to figure out if something else is causing your symptoms. 

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Article Sources
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  1. Drexler M, Institute of Medicine (US). What you need to know about infectious disease. Washington (DC): National Academies Press (US); 2010. I, How Infection Works. 

  2. National Human Genome Research Institute. Bacteria

  3. Centers for Disease Control and Prevention. About parasites. Updated September 18, 2020.

  4. Cleveland Clinic. Malaria. Updated August 20, 2020.

  5. Centers for Disease Control and Prevention. Aspergillosis. Updated December 9, 2020.

  6. Demain AL, Martens E. Production of valuable compounds by molds and yeasts. J Antibiot (Tokyo). 2017;70(4):347-360. doi:10.1038/ja.2016.121

  7. Harvard Medical School. Prion diseases

  8. Cleveland Clinic. Infectious diseases. Updated February 27, 2018.

  9. Cleveland Clinic. Shingles. Updated February 17, 2020. 

  10. National Institutes of Health. What infections can affect pregnancy? Updated January 31, 2017.

  11. van Seventer JM, Hochberg NS. Principles of infectious diseases: transmission, diagnosis, prevention, and control. International Encyclopedia of Public Health. 2017;22-39. doi:10.1016/B978-0-12-803678-5.00516-6

  12. Centers for Disease Control and Prevention. E. coli (Escherichia coli): Questions and answers. Updated December 1, 2014.

  13. Trident Health System. Ouch, what bit me? 10 common bug bites. July 10, 2018.

  14. Harvard Medical School. If you are at higher risk. How to reduce risk of infection and what to do if you get sick. Updated July 11, 2021.

  15. Merz B. Harvard Medical School. How to prevent infections.