Infectious diseases

Infectious diseases are disorders caused by organisms – such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies.

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.

Symptoms

Each infectious disease has its own specific signs and symptoms. General signs and symptoms common to a number of infectious diseases include:

  • Fever
  • Diarrhoea
  • Fatigue
  • Muscle aches
  • Coughing

When to see a doctor

Seek medical attention if you:

  • Have been bitten by an animal
  • Are having trouble breathing
  • Have been coughing for more than a week
  • Have severe headache with fever
  • Experience a rash or swelling
  • Have unexplained or prolonged fever
  • Have sudden vision problems

Causes

Infectious diseases can be caused by:

  • Bacteria. These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.
  • Viruses. Even smaller than bacteria, viruses cause a multitude of diseases — ranging from the common cold to AIDS.
  • Fungi. Many skin diseases, such as ringworm and athlete’s foot, are caused by fungi. Other types of fungi can infect your lungs or nervous system.
  • Parasites. Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be transmitted to humans from animal feces.

Direct contact

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:

  • Person to person. A common way for infectious diseases to spread is through the direct transfer of bacteria, viruses or other germs from one person to another. This can occur when an individual with the bacterium or virus touches, kisses, or coughs or sneezes on someone who isn’t infected.These germs can also spread through the exchange of body fluids from sexual contact. The person who passes the germ may have no symptoms of the disease, but may simply be a carrier.
  • Animal to person. Being bitten or scratched by an infected animal — even a pet — can make you sick and, in extreme circumstances, can be fatal. Handling animal waste can be hazardous, too. For example, you can acquire a toxoplasmosis infection by scooping your cat’s litter box.
  • Mother to unborn child. A pregnant woman may pass germs that cause infectious diseases to her unborn baby. Some germs can pass through the placenta. Germs in the vagina can be transmitted to the baby during birth.

Indirect contact

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.

Insect bites

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.

Food contamination

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.

Risk factors

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:

  • You’re taking steroids or other medications that suppress your immune system, such as anti-rejection drugs for a transplanted organ
  • You have HIV or AIDS
  • You have certain types of cancer or other disorders that affect your immune system

In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.

Complications

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:

  • Human papillomavirus is linked to cervical cancer
  • Helicobacter pylori is linked to stomach cancer and peptic ulcers
  • Hepatitis B and C have been linked to liver 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.

Diagnosis

Your doctor may order lab work or imaging scans to help determine what’s causing your symptoms.

Laboratory tests

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.

  • Blood tests. A technician obtains a sample of your blood by inserting a needle into a vein, usually in your arm.
  • Urine tests. This painless test requires you to urinate into a container. To avoid potential contamination of the sample, you may be instructed to cleanse your genital area with an antiseptic pad and to collect the urine midstream.
  • Throat swabs. Samples from your throat, or other moist areas of your body, may be obtained with a sterile swab.
  • Stool sample. You may be instructed to collect a stool sample so a lab can check the sample for parasites and other organisms.
  • Spinal tap (lumbar puncture). This procedure obtains a sample of your cerebrospinal fluid through a needle carefully inserted between the bones of your lower spine. You’ll usually be asked to lie on your side with your knees pulled up toward your chest.

Imaging scans

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.

Biopsies

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.

Treatment

Knowing what type of germ is causing your illness makes it easier for your doctor to choose appropriate treatment.

Antibiotics

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.

Antivirals

Drugs have been developed to treat some, but not all, viruses. Examples include the viruses that cause:

  • HIV/AIDS
  • Herpes
  • Hepatitis B
  • Hepatitis C
  • Influenza

Antifungals

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.

Anti-parasitics

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.

Alternative medicine

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:

  • Cranberry
  • Echinacea
  • Garlic
  • Ginseng
  • Goldenseal
  • Vitamin C
  • Vitamin D
  • Zinc

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.