Respiratory Infections Lesson: Causes, Symptoms & Key Concepts

Lesson Overview

• Streptococcal Pharyngitis and Scarlet Fever
• Corynebacterium diphtheriae and Diphtheria
• Otitis Media and Sinusitis
• The Common Cold
• Pertussis (Whooping Cough)
• Pneumococcal Pneumonia
• Atypical Pneumonia (Mycoplasma pneumoniae)
• Legionnaires' Disease
• Influenza
• Hantavirus Pulmonary Syndrome
• Respiratory Syncytial Virus (RSV)
• Latent Infections and Dormancy

Respiratory infections are illnesses that affect the respiratory tract, which is divided into the upper (nose, throat, sinuses, ears) and lower (trachea, bronchi, lungs) portions. These infections can be caused by a variety of pathogens – including bacteria, viruses, and occasionally fungi – and range from mild common colds to life-threatening pneumonias. 

Understanding the pathogens, the diseases they cause, and how we diagnose and treat them is crucial in biology and medicine. Below, we explore key respiratory infections, their causes, symptoms, and treatments in a structured way, emphasizing the reasoning behind these facts.

Streptococcal Pharyngitis and Scarlet Fever

Streptococcus pyogenes (Group A Streptococcus, GAS) is a bacterium that commonly causes strep throat (streptococcal pharyngitis). In some cases, certain strains of this bacterium produce additional toxins that lead to Scarlet Fever, characterized by a distinctive rash. Both conditions primarily affect the upper respiratory tract (throat) and are spread by respiratory droplets.

• Strep throat presents with sore throat, fever, swollen tonsils, and is treated with penicillin or amoxicillin.
• Scarlet fever features a sandpaper-like rash and a "strawberry tongue" caused by erythrogenic toxins from phage-infected bacteria.
• Early antibiotic treatment prevents complications like rheumatic fever.

Corynebacterium diphtheriae and Diphtheria

Corynebacterium diphtheriae is a Gram-positive rod (club-shaped bacterium) that causes diphtheria, a serious upper respiratory infection. Diphtheria usually infects the throat (pharynx), and toxigenic strains release a powerful exotoxin that can cause severe systemic damage​. Diphtheria is now rare in many countries due to vaccination, but it has historically been one of the most dangerous respiratory infections.

• Infection causes a thick gray membrane in the throat, sore throat, and neck swelling.
• The toxin affects distant organs, potentially leading to paralysis or heart failure.
• Treated with antitoxin and antibiotics like penicillin or erythromycin.
• Prevented by vaccination.

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Otitis Media and Sinusitis

Otitis media is a middle ear infection, and rhinosinusitis is a sinus infection. These are upper respiratory tract infections often occurring after or alongside a common cold. They are typically bacterial and cause symptoms like ear pain or sinus pressure. Several different bacteria can cause these infections, sometimes together.

• Common bacteria: Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus.
• Symptoms include ear pain, sinus pressure, congestion, and fever.
• Diagnosed clinically; treated with amoxicillin or broader antibiotics if needed.

The Common Cold

The common cold is a mild upper respiratory infection usually caused by viruses. It is not bacterial, hence antibiotics do not help. Colds are among the most frequent illnesses in humans, typically causing nasal congestion, runny nose, sore throat, sneezing, and cough. Fever is usually mild or absent.

• Causes sore throat, runny nose, sneezing, mild cough.
• Spreads via droplets or contaminated surfaces.
• Treated symptomatically; antibiotics are not effective.

Pertussis (Whooping Cough)

Pertussis, commonly known as whooping cough, is a highly contagious infection of the lower respiratory tract caused by the bacterium Bordetella pertussis. It is known for uncontrollable, forceful coughing fits that can make it hard to breathe; the "whoop" is the gasping sound patients (often children) make when inhaling after a coughing spell. 

Pertussis can be especially dangerous for infants and young children, sometimes leading to death by cyanosis (a blue tint to the skin from lack of oxygen)​

• Treated with macrolide antibiotics; most effective early.
• Prevented through vaccination.

Pneumococcal Pneumonia

Pneumonia is an infection of the lungs (lower respiratory tract). One of the most common causes of bacterial pneumonia is Streptococcus pneumoniae, often called the pneumococcus. 

Pneumococcal pneumonia is sometimes termed "typical pneumonia" and classically causes a lobar pneumonia (affecting a large section of one lung). It is a leading cause of community-acquired pneumonia, especially in young children and the elderly.

• Symptoms: high fever, rust-colored sputum, chest pain, and shortness of breath.
• Diagnosed via chest X-ray and lab tests.
• Treated with cephalosporins, macrolides, or fluoroquinolones.
• Vaccination helps prevent infection.

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Atypical Pneumonia (Mycoplasma pneumoniae)

Not all pneumonias follow the "typical" pattern. Mycoplasma pneumoniae is a unique bacterium that causes an atypical pneumonia often called "walking pneumonia." This pneumonia tends to have milder symptoms that come on more gradually, and the causative agent is very different from typical bacteria – M. pneumoniae has no cell wall. 

Atypical pneumonia often affects adolescents and young adults, and people can often continue daily activities (hence "walking" pneumonia, because patients are not bedridden as often as with typical pneumonia).

• Symptoms include persistent dry cough and low-grade fever.
• Not treatable with penicillins; treated with macrolides or doxycycline.

Legionnaires' Disease

Legionella pneumophila is a Gram-negative bacterium that causes Legionnaires' disease, a form of atypical pneumonia often acquired from environmental water sources. An infamous outbreak in 1976 among American Legion convention attendees gave this disease its name. 

Legionnaires' disease typically affects the lower respiratory tract, causing a severe pneumonia with high fever, cough, and sometimes diarrhea or confusion. It can be serious, especially in older adults or those with weakened immune systems.

• Symptoms: high fever, cough, diarrhea, confusion.
• Diagnosed with urine antigen test.
• Treated with macrolides or fluoroquinolones.

Influenza

Influenza is a viral infection of the lower respiratory tract that causes "the flu" – a potentially severe illness characterized by high fever, body aches, cough, and fatigue. It is caused by influenza viruses, which are RNA viruses belonging to the family Orthomyxoviridae.

Influenza should not be confused with the common cold; flu symptoms are generally more severe and can lead to serious complications like pneumonia.

• Causes fever, body aches, dry cough, fatigue.
• Diagnosed clinically or with lab tests.
• Treated with antivirals like oseltamivir.
• Prevented by annual vaccination.

Hantavirus Pulmonary Syndrome

Hantavirus Pulmonary Syndrome (HPS) is a serious respiratory illness caused by certain hantaviruses, which are viruses carried by rodents. Unlike most other infections discussed, hantavirus infection is zoonotic – humans get it from contact with infected rodents or their droppings, not from other infected people. 

HPS became widely recognized in an outbreak in the U.S. in 1993. It primarily affects the lungs, causing them to fill with fluid (pulmonary edema), and has a high mortality rate.

• Early symptoms: muscle aches, headache, chills.
• Later symptoms: respiratory distress, fluid-filled lungs.
• Treated with supportive ICU care; no specific antiviral.

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Respiratory Syncytial Virus (RSV)

Respiratory Syncytial Virus (RSV) is a common respiratory virus that infects most people in early childhood. In older children and adults, it usually causes a mild cold-like illness. RSV is a leading cause of hospitalization for infants and can be life-threatening in that age group.

• Symptoms: wheezing, difficulty breathing, nasal congestion.
• Treated with oxygen and supportive care.
• Prevented by good hygiene and monoclonal antibodies for high-risk infants.

Latent Infections and Dormancy

A dormant infection (or latent infection) means the pathogen is present in the host but is inactive (not causing symptoms or detectable illness). During dormancy, the microbe may be effectively controlled by the host's immune system or be in a non-replicating state. Because it's not active, the host typically has no symptoms during this period and may not even know the pathogen is there.

• Dormant infections include TB and herpes viruses.
• Disease appears long after initial infection, not immediately.
• Dormancy allows pathogens to hide from the immune system.