Infectious Diseases

Infectious diseases are disorders caused by bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They may be harmless or even helpful, but under certain conditions, some organisms may cause disease.

Some infectious diseases can be transmitted from person to person. Some are passed on by bites from insects or animals. And others are gotten by eating contaminated food, water or drink, or being exposed to organisms in the environment.

Signs and symptoms differ 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 hospitalization.


Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and systematic 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:

  • Diarrhea
  • Fever
  • Coughing
  • Fatigue
  • Muscle aches

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Infectious diseases can be caused by:

  • Viruses cause a multitude of mild to severe diseases ranging from the common cold to AIDS.
  • These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.
  • Fungi is responsible for many skin diseases, such as ringworm and athlete’s foot. Other types of fungi can infect your lungs or nervous system.
  • Malaria is caused by a tiny parasite that is transmitted by a mosquito bite. Other parasites may be passed on 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.Infectious diseases can be spread 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 coughs, touches, kisses, or sneezes on someone who isn’t ill. These germs can also spread through the exchange of body fluids from sexual contact.
  • 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 child. Some germs can pass through the placenta. Germs in the vagina can be transferred 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.

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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

Germs can infect people also through contaminated food and water. This mechanism of transmission permits germs to be spread to many people through a single source.

Risk factors

While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn’t functioning properly. This may occur if:

  • You have HIV or AIDS
  • You’re taking steroids or other medications that suppress your immune system
  • You have certain types of cancer or other disorders that affect your immune system

When to see a doctor

See your doctor immediately if you notice any of the following:

  • bitten by an animal
  • difficulty breathing
  • sudden vision problems
  • rash or swelling
  • cough that persisted for more than a week
  • prolonged fever
  • severe headache with fever


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 connected to a long-term increased risk of cancer:

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


Infectious agents can enter your body through:

  • Sexual contact
  • Inhaling airborne germs
  • Skin contact or injuries
  • Ingestion of contaminated food or water
  • Tick, mosquito or bites from other insects

Follow these tips to decrease your risk of infecting yourself or others:

  • Wash your hands.Wash your hands before and after preparing food, before eating, and after using the toilet. And try not to touch your eyes, nose or mouth with your hands.
  • Get vaccinated.Immunization can significantly reduce your chances of contracting many diseases. Keep up to date on your recommended vaccinations.
  • Stay home when ill.Don’t go to work if you are vomiting, have diarrhea or have a fever. Don’t send your child to school if he or she has these signs and symptoms, either.
  • Prepare food properly.Keep counters and other kitchen surfaces clean when preparing meals. Cook foods to the proper temperature using a food thermometer to check for doneness. For ground meats, that means at least 160 F (71 C); for poultry, 165 F (74 C); and for most other meat, at least 145 F (63 C).
  • Practice safe sex.Always use condoms if you or your partner has a history of sexually transmitted infections.
  • Don’t share personal items.Use your own toothbrush, comb and razor. Avoid sharing drinking glasses or dining utensils.


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