Introduction to Bacterial, Viral/COVID-19, and Fungal Infections

Core Concepts

Bacteria: prokaryotes with a cell wall

Bacteria are single-celled organisms without a nucleus (prokaryotes). Their defining structure is a cell wall made of peptidoglycan — a mesh human cells lack, and the target of several major antibiotic classes. Bacteria are classified by shape (cocci, bacilli, spirilla), Gram stain (gram-positive bacteria have thick peptidoglycan that stains purple; gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane and stain pink), and oxygen needs (aerobes versus anaerobes). Many bacteria are harmless or helpful (normal flora); infection occurs when a pathogen invades tissue — causing strep throat, urinary tract infections, pneumonia, or wound infections.

Viruses: obligate intracellular parasites

Viruses are not cells. A virus is a small genome (DNA or RNA) wrapped in a protein coat (capsid), sometimes surrounded by a lipid envelope with surface proteins. Viruses cannot reproduce alone: they attach to a host cell, enter it, and commandeer the cell's machinery to copy their genome and build new particles. This obligate intracellular lifestyle is why antivirals are hard to design — the virus hides inside our own cells and uses our own equipment. Common viral infections include influenza, the common cold, herpes viruses, HIV, hepatitis, and COVID-19 — and antibiotics have no effect on them.

COVID-19: a viral infection that changed practice

COVID-19 is caused by SARS-CoV-2, an enveloped RNA virus spread mainly through respiratory droplets and aerosols when an infected person coughs, sneezes, or talks. The virus's spike protein binds receptors on human airway cells — the route of entry — and is the target of the mRNA vaccines and many antibody-based treatments. Illness ranges from no symptoms to severe respiratory failure, with the worst outcomes driven partly by an overactive inflammatory response (why some hospitalized patients receive anti-inflammatory drugs alongside antivirals). Because the virus mutates and guidance evolves, current recommendations must always be checked against up-to-date references.

Fungi: eukaryotes that are harder to target

Fungi are eukaryotes — their cells have nuclei and organelles like human cells — which makes them harder to kill selectively than bacteria. A key difference is the cell membrane: fungal membranes contain ergosterol, a sterol human membranes lack, and ergosterol is the target of several antifungal classes. Fungi grow as yeasts (single cells, like Candida) or molds (branching filaments, like Aspergillus). Many fungal infections are opportunistic, striking people with weakened immune systems (chemotherapy, HIV, immunosuppressants), which is why antifungals are common in hospitals and among immunocompromised patients.

How infections happen: the chain of infection

For infection to occur, several links must connect: a reservoir (where the pathogen lives), a way to leave it, a mode of transmission (droplets, airborne particles, contact, contaminated food or water, vectors), a portal of entry (respiratory tract, skin, mucous membranes), and a susceptible host. Break any link — hand hygiene, masks, isolation, wound care, vaccination — and transmission stops. Nurses enforce this chain daily through standard and transmission-based precautions.

Colonization versus infection, and culture basics

A person can carry an organism without being sick (colonization), while infection means the organism has invaded tissue and triggered a response; colonization often needs no treatment, infection does. Culture and sensitivity (C&S) testing grows the organism from a specimen (blood, urine, sputum, wound) and tests which drugs inhibit it. Because antibiotics started before collection can suppress the organism and produce false-negative results, specimens are ideally collected before therapy begins — a classic nursing principle protecting diagnostic accuracy.

Nursing considerations

• Infection prevention first: hand hygiene, standard precautions, and appropriate isolation are the foundation of every infection-related action.
• Person-first language: People have infections; they are not defined by them (say "a person with pneumonia," not "a pneumonia patient").
• Education: explain why the drug class matches the organism, why the full course matters, and when to report worsening symptoms.
• Antimicrobial stewardship: question whether an anti-infective is indicated ("Is this viral or bacterial? Is a culture available?") and follow institutional stewardship programs — scope, policy, and formularies vary by setting.

Clinical Scenario: Three Specimens, Three Answers

A nursing student reviews three lab reports during clinicals. The first is a urine culture growing a gram-negative rod; the preceptor explains that gram-negative bacteria have an outer membrane that changes which antibiotics reach them. The second is a positive influenza PCR — a viral test, not a culture; no antibiotic is ordered because antibiotics would do nothing for the flu. The third is a sputum report growing yeast from a patient with a weakened immune system; treatment will target the fungal cell membrane, not the bacterial cell wall.

The takeaway: the kind of pathogen determines the class of drug, and the lab result separates guessing from targeting.

Common Confusions