What are infectious diseases?

Infectious diseases are illnesses caused by microscopic bacteria, viruses, fungi, and parasites. These tiny organisms are all around us – and also on and inside of us! Our natural immune system provides a robust defense against infection, preventing outside pathogens from entering the body while keeping our normal healthy bacteria in their place. However, when dealing with trillions of viruses and bacteria, sometimes a few pathogens find a way through our defenses and make us sick.

Many common infections are caused by microbes that evade our immune system to cause illness, including influenza (the “flu”) and pneumococcal pneumonia. Vaccines can help boost our immune defenses to protect us against many of these types of microscopic invaders (see section on “Vaccines” for more information).

Other infections directly attack the host immune system, such as HIV. Untreated HIV destroys the very cells that are meant to defend us against infection, over time leaving the body defenseless against other potentially deadly diseases. While we cannot yet cure HIV, once-a-day treatment options effectively and safely control the virus and allow the immune defenses to rebuild.

Perhaps surprisingly, most infections are caused by our own bacteria that end up in the wrong part of the body. For instance, urinary tract infections (UTIs) are caused by skin or gut bacteria that work their way into the bladder. People who are unable to fully empty their bladder and thereby “flush out” these bacteria are at particularly high risk for UTIs. Some lung infections are caused by normal mouth bacteria that can enter the lungs during aspiration and cause pneumonia. Intact skin is one of our best defenses against outside bacteria, and breaks in the skin caused by trauma or surgery can put people at risk for wound and bloodstream infections.

What do infectious diseases doctors do?

What are infectious diseases doctors? And why did we decide to devote our careers to the study and treatment of them? To be an expert in infectious diseases is to understand how pathogens cause disease and how best to treat infections when they develop. We need to know what can cause our immune defense to break down, predict what type of organism might take advantage of that failing, be familiar with the latest diagnostic tools available to identify offending pathogens, and then determine which antimicrobials are best and safest to treat an infection in that particular patient.

The field of infectious diseases is a fascinating specialty that never ceases to both amaze and challenge. In the clinic we help our patients manage chronic infections and protect against new disease. In the hospital we treat the sickest of the sick and help them survive life and limb-threatening illnesses. Every week we encounter unique situations, perhaps an atypical presentation of a common disease or an uncommon disease altogether. Oftentimes we are called upon as medical detectives to help solve the mystery of a persistent fever or a chronic cough or to interpret an unexplained laboratory finding.

Someone once said “to know syphilis is to know medicine”, referring to the fact that syphilis infection can masquerade as a myriad of different medical conditions, often fooling physicians for years. Sometimes even when the pathogen is known, treatment can be extraordinarily challenging, for in the era of drug-resistant bacteria, our precious stocks of antimicrobial agents are quickly losing their efficacy. New viruses are constantly threatening our world and challenging our diagnostic skills. Just in the past decade we have seen the emergence and re-emergence of H1N1 “swine flu”, MERS, and Zika, as well as the 2014 Ebola outbreak in West Africa. Who knows what infectious disease challenge we will face next.