PCAT Exam: Biological Processes - Infectious Diseases & Prevention

Infectious diseases are caused by the spread of microorganisms from one person to another, either directly or indirectly. Direct contact involves that exchange of bodily fluids or droplets between an infected person and another person. Indirect contact involves airborne spread or touching a contaminated object. Individuals with compromised immune systems are often more susceptible hosts than healthy individuals. The spread of infectious diseases can be prevented by thorough sanitization and disinfection, such as hand washing, vaccination, and the use of disinfectants while cleaning surfaces.

Immunity and Serology 
Serology is the study of blood serum, the clear fluid that separates when blood clots, and how it relates to the immune system. The body’s immune system is the network of cells, tissues, and organs that helps fight off infections. Immunity is the ability of an organism to use specific antibodies to fight an infection or toxin. Serology includes identifying these antibodies present in blood serum and investigating problems with the immune system.

Three important serology tests include those for immunoglobulins (proteins responsible for antibody activity), rheumatoid factor (involved in certain types of arthritis), and human leukocyte antigen (HLA) typing (which determines organ, tissue, and bone marrow transplant compatibility).

AIDS and Immune Disorders 
Immunodeficiency disorders occur when the body’s immune system is unable to defend itself against foreign cells that can cause infection. This can cause unusual, prolonged, and/or frequent infections or cancers. There are two types of immunodeficiency disorders: primary and secondary.

Primary disorders are generally hereditary and present at birth. Secondary disorders develop later in life and result from the use of certain drugs or from another disorder. Acquired immune deficiency syndrome (AIDS) is a secondary disorder that develops from the human immunodeficiency virus (HIV). AIDS develops from HIV when a specific set of T cells, CD4+, from the immune system are depleted. The absence of these cells prevents the body from effectively fighting infections or killing cancerous cells. Immunodeficiency disorders can also include autoimmune disorders, which occur when the body’s immune system attacks itself as if it were a foreign pathogen.

Antimicrobial Medications and Drugs 
An antimicrobial medication or drug is used to treat a microbial infection. It can be antibiotic, antifungal, antiprotozoal, or antiviral. These drugs work by penetrating the cell wall of the microorganism and then disrupting the inside of the cell. They work to inhibit microbial growth and reproduction. The therapeutic index of a drug is a measure of its relative toxicity to a patient. It is calculated by taking the lowest dose that is toxic to a patient and dividing it by the dose typically used for therapy. Drugs that have antimicrobial selective toxicity are more harmful to microorganisms than patients. Some strains of microorganisms can change and become antimicrobial-resistant. When this happens, the microorganisms are no longer harmed by the medications, so they continue to survive, multiply, and harm the patient.

Systemic Infectious Diseases 
Systemic infections are infections that occur in the bloodstream and therefore affect the whole body. As the infection is carried in the blood, it can affect multiple organs and tissues and cause multiple systemic infectious disease syndrome (MSIDS). Patients affected by MSIDS can have a variety of concurrent symptoms, making it hard to identify the source of the infection and thus also making it hard to treat the infection. The flu is an example of a systemic infection, and hypertension is an example of a systemic disease.

Infectious Diseases Affecting the Cardiovascular, Respiratory, Lymphatic and Nervous Systems 
Infections of the cardiovascular system affect the blood, blood vessels, and the heart. Septicemia is the general term given to a microbial infection of the blood and blood vessels. If this infection reaches the heart valves, it results in endocarditis. Generally, this can be treated with antibiotics, but if there’s too much damage to the heart, surgery may be needed. Common infections of the respiratory tract are the common cold and flu. Bacterial infections are less common than viral infections in the respiratory system. These affect the sinuses, throat, airways, or lungs. Pneumonia is an example of a bacterial infection of the lower respiratory tract. When microorganisms infect the lymphatic system, the lymph, lymph vessels, lymph nodes, and lymphoid organs—such as the spleen, tonsils, and thymus—are affected. Infectious lymphangitis occurs when viruses or bacteria invade the vessels of the lymphatic system through an infected wound. Infections of the central nervous system can be very serious, as they affect the brain and spinal cord. Brain abscesses and bacterial meningitis are caused by bacteria or fungi, while viral meningitis and encephalitis are caused by viruses.

Infectious Diseases Affecting the Digestive, Urinary, and Reproductive Systems 
When microorganisms enter the digestive tract, they cause gastrointestinal infections, which are an inflammation of the gastrointestinal (GI) tract that involves the stomach and the small intestine. Dehydration is the largest worry with GI infections, as the patient may not be absorbing enough water while affected by the virus, bacteria, or parasite. Infections of the urinary tract (UTIs) are most often caused by bacteria. They are often the result of bacteria from the large intestine entering the urethra and traveling up to the bladder. If they are not treated in a timely manner, the infection can also continue up to the kidneys and cause a serious infection. Symptoms of a kidney infection can include chills, fever, back pain, and nausea.

There are three types of reproductive tract infections: sexually-transmitted diseases, endogenous infections, and iatrogenic infections. Sexually-transmitted diseases (STDs), such as chlamydia, gonorrhea, and HIV, are transmitted from one person to another by bodily fluids that are part of the reproductive system.

Endogenous infections are caused by the abnormal growth of organisms that are normally present, such as bacterial vaginosis. Iatrogenic infection of the reproductive system occurs when microorganisms are introduced during an unsterile medical procedure. Serious reproductive infections may result in infertility.

Infectious Diseases Affecting the Skin and Eyes 
Although the skin provides a barrier from infection, it sometimes gets infected. Bacterial infections include cellulitis and impetigo. Viral infections include shingles, warts, and herpes simplex virus. These infections can start with a rash, itching, pain, and tenderness. Most can be treated with antibiotics. Fungal infections include yeast infections, athlete’s foot, and ringworm. They often occur when there is a cut on the skin’s surface and the body’s immune system is weakened. Microorganisms can also affect the surface or interior of the eyes. The most common eye infection is conjunctivitis, or pink eye, which is caused by the viruses and bacteria that cause the common cold. Bacterial keratitis is an infection of the cornea. When microorganisms reach the interior of the eyes, pain is not usually felt, but vision starts to deteriorate.