Overview 

Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi, or parasites. Many organisms live in and on our bodies. They're generally harmless or even helpful. However, under certain conditions, some organisms may cause disease.

Some infectious diseases can be passed from person to person. Insects or other animals transmit some, but you may get others by consuming contaminated food or water or being exposed to environmental organisms.

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 need hospitalization.

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.

Signs and Symptoms of Infectious Disease

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

• Fever
• Diarrhea
• 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 an 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 an animal with the infection. Infectious diseases can be spread through direct contact such as:

• Person to person. Infectious diseases commonly spread through the direct transfer of bacteria, viruses, or other germs from one person to another. This can happen when an individual with the bacterium or virus touches, kisses, coughs, or sneezes on someone who isn't infected.

  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 get 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 or through breast milk. Germs in the vagina can also be transmitted to the baby during birth.

Indirect contact

Disease-causing organisms can also 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, 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. Deer ticks may carry the bacterium that causes Lyme disease.

Food contamination

Disease-causing germs can also infect you through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. For example, Escherichia coli (E. coli) is a bacterium present in or on certain foods — such as an undercooked hamburger or unpasteurized fruit juice.

Preventing Infectious Disease

Keep immunizations up-to-date

• Follow recommended immunizations for children and adults.
• Remember, pets need their shots too!

Wash your hands often, especially:

• After using the bathroom
• Before preparing food or eating
• After changing diapers
• After blowing your nose and/or using your hands when sneezing or coughing
• After caring for a sick person
• After playing with or handling your pet

Prepare/handle food carefully

• Wash your hands before and after handling food.
• Keep hot foods hot and cold foods cold until eaten or cooked.
• Be sure temperature controls in refrigerators and freezers are working properly.
• Wash counters, cutting boards, and utensils with soap and hot water, especially after preparing eggs, poultry, or other meats. Use separate cutting boards for meat.
• Wash fresh fruits and vegetables before eating.
• Cook meat, poultry, and eggs thoroughly. A meat thermometer is the best way to ensure food is thoroughly cooked.
• Refrigerate leftovers as soon as possible after meals.

Use antibiotics only for infections caused by bacteria

• Antibiotics are not useful in treating infections caused by viruses (for example, antibiotics will not shorten the course of a cold).
• Take antibiotics exactly as prescribed. Complete the full course of treatment.
• Never self-medicate with antibiotics or share them with family or friends.

Report to your doctor any rapidly worsening infection or any infection that does not get better after taking a course of antibiotics if prescribed

Be careful around all wild animals and domestic animals unfamiliar to you.

• After any animal bite, cleanse the wound with soap and water and seek immediate medical care.

Avoid insect bites

• Use insect repellants on skin and clothing when in areas where ticks or mosquitoes are common.
• If you have visited wooded or wilderness areas and become sick, tell your doctor all the details in order to help diagnose both rare and common illnesses quickly.

Stay alert to disease threats when you travel or visit underdeveloped countries.

• Get all recommended traveler’s immunizations.
• Use protective medications for travel, especially to areas with malaria.
• Don’t drink untreated water, especially while hiking or camping.
• Tell your doctor where you've been if you become ill when you return home.

Develop healthy habits such as eating well, getting enough sleep, exercising, and avoiding tobacco and illegal drug use.

  Check out The National Foundation of Infectious Diseases for more information on specific infectious diseases.