They’re normally harmless or even helpful, but under certain conditions, some organisms may cause disease.
Some infectious diseases can be passed from person to person. Some are transmitted by bites from insects or animals. And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.
Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue. Mild infections may respond to rest and home remedies, while some life-threatening infections may require hospitalisation.
Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from most infectious diseases.
Each infectious disease has its own specific signs and symptoms. General signs and symptoms common to a number of infectious diseases include:
When to see a doctor
Seek medical attention if you:
Infectious diseases can be caused by:
An easy way to catch most infectious diseases is by coming in contact with a person or animal who has the infection. Three ways infectious diseases can be spread through direct contact are:
Disease-causing organisms also can be passed by indirect contact. Many germs can linger on an inanimate object, such as a tabletop, doorknob or faucet handle.
When you touch a doorknob handled by someone ill with the flu or a cold, for example, you can pick up the germs he or she left behind. If you then touch your eyes, mouth or nose before washing your hands, you may become infected.
Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks — to move from host to host. These carriers are known as vectors. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that causes Lyme disease.
Another way disease-causing germs can infect you is through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. E. coli, for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unpasteurized fruit juice.
While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn’t working properly. This may occur if:
In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.
Most infectious diseases have only minor complications. But some infections — such as pneumonia, AIDS and meningitis — can become life-threatening. A few types of infections have been linked to a long-term increased risk of cancer:
In addition, some infectious diseases may become silent, only to appear again in the future — sometimes even decades later. For example, someone who’s had a chickenpox infection may develop shingles much later in life.
Your doctor may order lab work or imaging scans to help determine what’s causing your symptoms.
Many infectious diseases have similar signs and symptoms. Samples of your body fluids can sometimes reveal evidence of the particular microbe that’s causing your illness. This helps your doctor tailor your treatment.
Imaging procedures — such as X-rays, computerized tomography and magnetic resonance imaging — can help pinpoint diagnoses and rule out other conditions that may be causing your symptoms.
During a biopsy, a tiny sample of tissue is taken from an internal organ for testing. For example, a biopsy of lung tissue can be checked for a variety of fungi that can cause a type of pneumonia.
Knowing what type of germ is causing your illness makes it easier for your doctor to choose appropriate treatment.
Antibiotics are grouped into “families” of similar types. Bacteria also are put together in groups of similar types, such as streptococcus or E. coli.
Certain types of bacteria are especially susceptible to particular classes of antibiotics. Treatment can be targeted more precisely if your doctor knows what type of bacteria you’re fighting.
Antibiotics are usually reserved for bacterial infections, because these types of drugs have no effect on illnesses caused by viruses. But sometimes it’s difficult to tell which type of germ is at work. For example, some types of pneumonia are caused by viruses while others are caused by bacteria.
The overuse of antibiotics has resulted in several types of bacteria developing resistance to one or more varieties of antibiotics. This makes these bacteria much more difficult to treat.
Drugs have been developed to treat some, but not all, viruses. Examples include the viruses that cause:
Topical antifungal medications can be used to treat skin or nail infections caused by fungi. Some fungal infections, such as those affecting the lungs or the mucous membranes, can be treated with an oral antifungal. More severe internal organ fungal infections, especially in people with weakened immune systems, may require intravenous antifungal medications.
Some diseases, including malaria, are caused by tiny parasites. While there are drugs to treat these diseases, some varieties of parasites have developed resistance to the drugs.
A number of products have been purported to help fend off common illnesses, such as the cold or flu. While some of these substances have appeared promising in early trials, follow-up studies may have had negative or inconclusive results. More research needs to be done.
Some of the substances that have been studied for preventing or shortening the duration of infection include:
Check with your doctor before trying any products that promise to boost your immune system or chase colds and other illnesses away. Some of these products may cause allergic reactions or interact adversely with other medications you may be taking.