What are infectious diseases?

Infectious diseases can be caused by many pathogens, including bacteria, viruses, fungi, and parasites that may cause illness and disease. For humans, transmission of pathogens may occur in a variety of ways: spread from person-to-person by direct contact, water or foodborne illness or aerosolization of infected particles in the environment and through insects (mosquitoes) and ticks.

Signs and symptoms and treatment of infectious diseases depend on the host and the pathogen.

Who is most at risk for getting infectious diseases?

Anyone can get an infectious disease. People with a compromised immune system (an immune system that doesn’t work at full strength) have greater risk for certain types of infections. Those at higher risk include:

  • People with suppressed immune systems, such as those going through cancer treatment or who have recently had an organ transplant
  • Those who are unvaccinated against common infectious diseases
  • Healthcare workers
  • People traveling to at-risk areas where they may be exposed to mosquitoes that carry pathogens such as malaria, dengue virus and Zika viruses.

How common are infectious diseases?

Infectious diseases are extremely common worldwide. Some infectious diseases strike more often than others. For instance, in the United States, 1 out of every 5 people is infected with the influenza (flu) virus each year.

What complications are associated with infectious diseases?

Many infectious diseases cause complications. These can range from mild to severe. For some conditions, complications may include wheezing, skin rash, or extreme fatigue. Mild complications usually disappear as the infection resolves.

Certain infectious diseases may cause cancer. These include hepatitis B and C (liver cancer), and human papillomavirus (HPV) (cervical cancer).

What are the symptoms of infectious diseases?

Symptoms of infectious disease are particular to the type of disease. For example, symptoms of influenza include:

Other infectious diseases, such as Shigella, cause more serious symptoms, including:

  • Bloody diarrhea
  • Vomiting
  • Fever
  • Dehydration (lack of fluid)
  • Shock

You may experience one or several symptoms of an infectious disease. It’s important to see a doctor if you have any chronic (ongoing) symptoms or symptoms that get worse over time.

What causes infectious diseases?

Infectious diseases in humans are caused by microorganisms including:

  • Viruses that invade and multiply inside healthy cells
  • Bacteria, or small, single-celled organisms capable of causing disease
  • Fungi, which include many different kinds of fungus
  • Parasites, which are organisms that live inside host bodies causing sickness

Infectious diseases spread in multiple ways. In many cases, direct contact with a sick individual, either by skin-to-skin contact (including sexual contact) or by touching something another person touches, transmits the disease into a new host. Contact with body fluids, such as blood and saliva, also spreads infectious diseases.

Some diseases spread through droplets discharged from a sick person’s body when they cough or sneeze. These droplets linger in the air for a short period of time, landing on a healthy person’s skin or inhaled into their lungs.

In some cases, infectious diseases travel through the air for long periods of time in small particles. Healthy people inhale these particles and later become sick. Only certain diseases spread with airborne transmission, including tuberculosis and the rubella virus.

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