Infection is the invasion of the body by disease-causing organisms, known collectively as pathogens. Many pathogens are microorganisms like bacteria, while others are infectious agents like viruses, fungi like ringworm, or larger organisms like parasitic worms (e.g., roundworms, pinworms, etc.).

Classification of Infections

Infections are generally classified by the causative agent and signs and symptoms produced, if any. Symptomatic infections are active and apparent; active infections that do not produce noticeable symptoms are called inapparent or silent. An inactive infection is called latent.

A short-term infection is acute; a long-term infection is chronic. An infection hidden and only revealed by a secondary condition or manifestation is known as occult.

Sign and symptoms

Signs and symptoms of infection depend on the type of disease. Some signs and symptoms affect the entire body in a general way (e.g., fatigue, weight loss, fever, body aches, etc.) while other signs and symptoms are localized (e.g., a rash, a runny nose, or a sore throat).

Bacterial vs. viral infections

Bacterial and viral infections are often difficult to distinguish because they can have similar signs and symptoms. It is important to determine the type infection in these cases because bacterial infections respond to antibiotics while viral infections do not.

  • Bacterial infections usually begin with localized redness, swelling, and pain at the site of an injury or specific part of the body. For example, a cut that becomes red, hot, and filled with pus is most likely infected. Left untreated, bacterial infections can spread and even develop into septic shock or other life-threatening condition.
  • Viral infections usually involve several parts of the body at one time (i.e., sinus congestion, fever, and body aches). Influenza and the common cold are both viral infections.

How Diseases Spread

The infection process begins when a pathogen enters the body in a way that allows it to colonize, multiply, and overpower the body’s immune response, causing disease. The following pathogens are capable of causing diseases (those listed among others) in humans:

The body has a series of natural defenses that prevent pathogens from entering the body and spreading. The first line of defense is intact skin and mucous membranes in the mouth, nose, and eyes that keep pathogens outside the body. First aid responders and emergency medical personnel can fortify the body’s natural barriers by using personal protective equipment. If these barriers fail and the pathogen gets into the body, the immune system begins working to fight the invading pathogens.

The main tools of the immune system are white blood cells and Y-shaped proteins called antibodies. When white blood cells identify a pathogen, they surround it and release antibodies to fight the infection. To read more about this process, click this link.

When the combination of protective barriers and pathogen elimination by the immune system is overwhelmed, infection occurs. This can range from mild to severe and acute to chronic. Some infections are even life-threatening.

The building blocks of infection

It isn’t easy for a healthy person to become infected with a disease. In fact, we are constantly fighting off invading pathogens around, most of the time with no ill effects. For any disease to infect you, all four of the following conditions must be present:

  • A pathogen must be present.
  • A sufficient quantity of the pathogen must be present.
  • You must be susceptible to the pathogen.
  • The pathogen must enter the body through the correct entry site to cause infection.

Think of these conditions as the building blocks of infection and disease.

One of the most important of these conditions is the correct entry site.


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