What they are: Non-living, submicroscopic, proteins.

How they infect: Exposure occurs through consumption of meat tainted with prions, through contaminated medical equipment, or by receiving corneas or organs from infected individuals. These infectious proteins cause normal proteins in the brain to fold abnormally.

Diagnosis: MRI, cerebrospinal fluid analysis, electroencephalogram (EEG) which analyzes brain waves, and blood tests.

Treatment: There is no cure for prion diseases, but medications can slow the progression of the disease.

Prevention: Properly cleaning and sterilizing medical equipment may prevent the spread of the disease. Educating prion-infected individuals about not donating organs or tissue.

Example: Creutzfeldt-Jakob disease (CJD) results in dementia, walking difficulties, hallucinations, confusion, and death.

What they are: Non-living microscopic agents made up of an outer protein shell, called a capsid, and either DNA or RNA.

How they infect: The method of infection depends on the type of virus. Infection may occur through touch, saliva, blood, or sexual contact. Viruses cannot replicate on their own. To reproduce, they infect cells and take over the host cells’ protein synthesis processes. A side effect is that host cells and tissues are destroyed.

Diagnosis: Physical examination and clinical laboratory testing.

Treatment: TAntivirals may be used under certain circumstances. Over-the-counter treatments do not treat the infection but can sometimes relieve symptoms. These include pain relievers, decongestants, and cough syrups.

Prevention: Vaccines and proper personal hygiene.

Example: The influenza virus, the flu, affects the respiratory system. Symptoms include runny or stuffy nose, itchy or sore throat, cough, congestion, slight body aches or a mild headache, sneezing, watery eyes, a low-grade fever, and fatigue.

What they are: Living, microscopic, unicellular, prokaryotic organisms.

How they infect: The method of infection depends on the type of bacteria. Infection may occur through an orifice (nose, eyes, mouth), through a wound, by ingesting contaminated food or water, through inhalation, through sexual contact or may result due to an imbalance in our normal flora. Bacteria can invade cells, cause tissue damage, and produce harmful toxins.

Diagnosis: Physical examination and clinical laboratory testing.

Treatment: Antibiotics can be used to treat bacterial infections.

Prevention: Vaccines, maintenance of sanitary conditions, and proper personal hygiene.

Example: Mycobacterium tuberculosis causes tuberculosis (TB). The symptoms of TB depend on where in the body the bacteria are growing. Usually, TB bacteria grow in the lungs where they cause a bad cough that lasts three weeks or longer, pain in the chest, and coughing up blood or the phlegm from deep inside the lungs. Other symptoms include weakness, fatigue, weight loss, no appetite, chills, and fever.

What they are: Living, microscopic, single-celled, eukaryotic, animal-like organisms.

How they infect: Infection typically results through the ingestion of food or water that has been contaminated with feces from an infected organism. Protists infect the digestive tract, blood, or organs of the body depriving a host of essential nutrients found in food. They can also cause tissue and organ damage.

Diagnosis: Microscopic examination of fecal material to look for the presence of the protists.

Treatment: Some infections can be treated with antiprotozoal medication.

Prevention: Maintenance of sanitary conditions and proper personal hygiene.

Example: Giardia lamblia causes giardiasis. Giardia parasites live in the intestines of people and animals. Before the parasites are passed in the stool, they become encased within hard shells called cysts, which allows them to survive outside the intestines for months. Once inside a host, the cysts dissolve and the parasites are released.

What they are: Living, multicellular, eukaryotic worms. Both microscopic and macroscopic varieties exist.

How they infect: The method of infection depends on the type of worm. Infection may occur through consumption of contaminated foods or water or by being bit by an infected insect. Helminths deprive the host of essential nutrients found in food and can cause tissue and organ damage.

Diagnosis: Microscopic examination of fecal material or blood to look for the presence of the worms.

Treatment: Deworming medications.

Prevention: Varies depending on the type of worm, but includes avoiding:

• Exposure to food or water contaminated with feces from a person or animal infected with a worm
• Ingestion of meat from an animal infected with a worm
• Insect bites

Example: Taenia, a tapeworm infection, results in nausea, loss of weight, abdominal pain, loss of appetite, diarrhea, weight loss, and malnutrition. Invasive infections result in fever, cystic masses or lumps, allergic reactions to the larvae, bacterial infections, and neurological symptoms including seizures.

What they are: Living, multicellular, eukaryotic organisms. Fungi include yeasts, mildews, molds, and mushrooms. Both microscopic and macroscopic varieties exist.

How they infect: Some forms infect when the spores of the fungi, the reproductive cells, are inhaled or land on a host. Infection can result in tissue damage.

Diagnosis: Microscopic examination of tissue taken from the infected area, or blood taken from the host, to look for the presence of the fungi.

Treatment: Antifungals.

Prevention: Avoiding contact with an infected host.

Example: Tinea pedis causes athlete’s foot. Symptoms include dryness, itching, stinging, burning, cracking and peeling skin and blisters between the toes or on the soles of the feet. Toenails may be thick, crumbly, ragged, discolored, or pulling away from the nail bed. Athlete’s foot thrives in thick, tight shoes that squeeze the toes together and create warm, moist areas between them. Damp socks and shoes and warm, humid conditions also favor the organisms’ growth.