Infection and Disease

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

Viral infections

Viral infections (click to enlarge)

Bacterial infections

Bacterial infections (click to enlarge)

Bacteria and viruses are everywhere. Bacteria, for example, constitute a large domain of prokaryotic microorganisms (approximately 5 x 1030 exist on earth—this is a biomass larger than all plants and animals combined). Viruses are tiny infectious agents that infect and replicate within humans, animals, plants, and microorganisms. There are millions of types of viruses, but only about 5,000 have been described in detail.

Bacterial and viral infections are often difficult to distinguish because they can have similar signs and symptoms. The following descriptions may help distinguish whether an infection is bacterial or viral.

  • 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.

To read more about the differences between bacteria and viruses and the infections they cause, review the illustrations above or go to the Mayo Clinic, microbeworld, and WebMD.

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 be passed to you using the correct mode of transmission.

One of the most important of these conditions is the correct mode of transmission. Different modes of transmission follow:

  • Airborne transmission: Pathogens coughed, sneezed, or breathed into the air are able to survive long enough to pass to others.
  • Direct contact: Pathogen transmission occurs as the result of body-to-body physical contact, including blood-to-blood contact, oral contact (i.e., kissing, etc.), and sexual contact.
  • Droplet transmission: Droplets from a cough or sneeze of an infected person are inhaled through the mouth or nose of people within about 3 feet of the cough or sneeze
  • Indirect contact: Pathogens are spread by contact with surfaces or objects contaminated with infected blood, body fluids, stool, etc. This mode includes fecal-oral transmission (infected stool is swallowed by eating contaminated food, drinking contaminated water, touching contaminated hands to mouth after going to the toilet, changing diapers, touching sewage, etc.)
  • Latrogenic transmission: Pathogens transmitted through medical procedures such as injections, operations, insertion of medical equipment into the body, etc.
  • Vector transmission: Disease transmission that occurs because of the bite or sting of an organism that does not cause disease but transmits pathogens from one host to another
  • Vertical transmission: Transmission of disease from mother to offspring during pregnancy or childbirth or, more rarely, during breastfeeding.

Click the link to watch a great discussion of the “5-second rule,” a type of indirect contact.