What you need to know about infectious disease

Some doctors would tell you that everything is infectious—in some ways, even certain psychiatric diseases are contagious! In medical terms, however, infectious diseases are those illnesses caused by external organisms—including viruses, bacteria, parasites, and some less-common vectors such as fungi and sporozoites. Some of these are easily transmitted between humans, while others are contracted from animals. They are external infections, external invaders that can cause damage.

Most people who contract such a disease won’t need to see an infectious disease (ID) specialist. Specialists usually deal with special or unusual infections, or cases where standard treatments aren’t working. An example might be when a patient’s condition is not responding to normal antibiotic treatment—an ID specialist will assess the resistance of the bacterial infection to antibiotics, and choose a more appropriate antibiotic or drug regime in complicated patients.

ID specialists work in a fascinating medical field—the domain of discovering diseases, diagnosing them, finding a treatment, and most importantly discovering how to prevent infection and arrest the spread of a disease.

When assessing means of treatment and control, it’s very important for an ID specialist to take into account three things: the potential transmissibility (how fast and easily a disease spreads), morbidity (how many people potentially become ill rather than just carry the infectious agent, and what the severity of the illness is) and mortality (how many people who become ill may die) of any given disease. The most alarming infectious diseases—which could be described as plagues—would be highly transmissible, cause many people to become sick, and have a high death toll.

Most diseases circulating in the world today are not combinations of those three dreaded factors—they are not highly transmissible while at the same time having a high rate of infectivity and causing death in high proportions. Influenza spreads extremely rapidly, although most people who contract the virus suffer only cold-like symptoms for a period of one to three weeks. HIV is transmitted only seldom via intimate contact, but it almost inevitably causes severe morbidity and fatality without treatment.

Some infectious diseases can be diagnosed clinically. The doctor is presented with a cluster of symptoms or a distinct visible symptom that may be readily diagnosed; the aspect of a chickenpox rash, for example, is easy to recognize—so you don’t have to isolate the virus or take blood to check for antibodies. You just know it when you see it.

At other times, you may see a patient who has a chronic cough or some prolonged fever, and it’s not clear what the infection is. You may need to use tests—sometimes attempting direct isolation of the organism when you find it in a place where it shouldn’t be, such as observing mycobacteria in the sputum that tells you a patient is suffering from tuberculosis.

Then you have indirect identification, not by isolating the organism itself, but by finding antibodies produced by the immune system—telling you that the immune system saw a certain organism and reacted against it.

Simply finding antibodies does not necessarily give you the answer, however, because we all carry antibodies to various kinds of infections that we may have recovered from in the past. Our bodies maintain a stock of antibodies to protect against any future possible reinfections of illnesses we’ve fought against before.

Sometimes doctors mistakenly diagnose a disease simply by not understanding what the test means— finding antibodies to a disease doesn’t necessarily mean that disease is creating the problem right now, it may be something from the past.

However, if we identify early forms of antibodies, or a rapid increase in the concentration of antibodies within a short time period, then we can be fairly certain that the immune system is reacting to a live infection. In such cases, you can connect the presence of antibodies to the disease of the patient.

The part of the world where you live is also an important consideration when making a diagnosis. An ID specialist should know the local incidence of certain diseases. If you’re in New York and somebody comes to you with a prolonged fever, you will not jump on the possibility of tuberculosis as you might here in Vietnam, because there are different incidences in different parts of the world.

This is the importance of an ID specialist, who should choose the right diagnostic tool, know how to interpret tests, and decide which will be the right medication to use, given the local epidemiology.

Infection and its consequences are in perpetual balance between the surrounding potential organisms that can affect us, and the ability of our immune systems to develop the means to handle and control them. This is why people with a weak immune system are more at risk, whether they are babies or elderly persons, or those with a background condition that affects the immune system—which includes poor nutrition. By keeping the immune system healthy and staying up to date with vaccines (including the annual flu vaccine) we are far better equipped to fight off diseases.