17.2 — Viruses

A virus outside a cell is a chemical, not an organism (Chapter 1.1). It has no metabolism, cannot move on its own, and cannot reproduce. What it has is the ability to get inside a cell that can do all of those things, and redirect it.

Which is exactly why antibiotics do nothing against them — there is no bacterial machinery to attack, because the virus is using yours.

How a virus works

Six steps, and every antiviral drug blocks one of them.

Attachment — the virus binds a specific receptor on the cell surface.

And that specificity determines everything about the disease. HIV binds CD4, so it infects helper T cells (Chapter 13.2). Rhinovirus binds a molecule on airway cells, so it causes colds. Rabies binds the acetylcholine receptor, so it travels along nerves.

Which is also why species barriers exist and why crossing them requires a receptor change.

Entry — by fusion or by being engulfed.

Uncoating — releasing the genetic material.

Replication — hijacking the cell's machinery.

Assembly.

Release — by bursting the cell, or by budding through the membrane, which lets the cell survive and keep producing.

The types

DNA viruses — generally more stable, because DNA copying has proofreading (Chapter 2.2).

Herpesviruses are the ones worth knowing, because they share a defining property: after the first infection they persist for life in a latent state and reactivate.

Herpes simplex — cold sores and genital herpes, latent in sensory ganglia (Chapter 15.13). Varicella zoster — chickenpox, then shingles decades later from the same ganglion (Chapter 11.8). Epstein–Barr — glandular fever, and strongly linked to multiple sclerosis (Chapter 13.6) and to several cancers. Cytomegalovirus — usually silent, and serious in pregnancy and in immunosuppression.

Hepatitis B (Chapter 17.10). HPV (Chapter 15.13).

RNA viruses — generally mutate far faster, because RNA copying has no proofreading.

Which is the single most consequential difference between the two groups. It is why influenza needs a new vaccine every year and measles does not; why HIV requires three drugs at once; and why coronaviruses produce variants.

Influenza, coronaviruses, HIV, measles, polio, rabies, dengue, hepatitis A, C and E, and the common cold viruses.

Retroviruses — RNA viruses that convert their genome to DNA and integrate it into yours. HIV.

And integration is why HIV cannot currently be cured: the provirus sits in the genome of long-lived cells, invisible to both the immune system and to drugs, and reactivates whenever treatment stops.

Why antivirals are harder than antibiotics

Because the virus uses your machinery, there is far less that is uniquely viral to attack.

So antivirals target the few virus-specific steps:

Viral enzymes — reverse transcriptase and protease in HIV; polymerase in hepatitis and influenza. Aciclovir is activated only by a viral enzyme, which is why it acts almost exclusively inside infected cells and is remarkably safe (Chapter 2.2).

Entry — blocking the receptor or the fusion step.

Release — oseltamivir blocks the enzyme influenza needs to detach from the cell surface.

And most antivirals must be started early, because they stop replication rather than repairing damage. Oseltamivir given within 48 hours shortens influenza by around a day; given later it does very little.

Where antiviral medicine has succeeded

And this is genuinely one of the strongest arguments for medical research.

Hepatitis C: from an incurable progressive liver disease to a cure rate above 95 percent with 8 to 12 weeks of well-tolerated tablets, within about a decade (Chapter 17.10).

HIV: from a diagnosis with a median survival of around ten years to a condition managed with one tablet a day and a near-normal life expectancy (Chapter 17.9).

Hepatitis B: suppressed indefinitely, and preventable by vaccine.

COVID-19: vaccines developed and deployed within a year, and effective oral antivirals within two.

And prevention has done more than treatment. Smallpox eradicated. Polio reduced by over 99 percent. Measles deaths down by over 80 percent since 2000 (Chapter 13.5).

The common cold, briefly

Over 200 viruses cause it, rhinoviruses accounting for around half.

Which is why there is no vaccine and no cure — you cannot immunise against 200 targets that change.

And why you get repeated colds: you are immune to the ones you have had, and there are plenty left.

What actually helps: rest, fluids, paracetamol or ibuprofen for symptoms, and time. What does not: antibiotics, which do nothing and cause harm.

Zinc lozenges started within 24 hours may shorten it slightly. Vitamin C does not prevent colds in the general population, though it may modestly shorten them and does appear to help in people under extreme physical stress.

And green or yellow nasal discharge does not indicate bacterial infection — it is neutrophil enzymes, and it occurs in ordinary viral colds. This single misconception drives a substantial fraction of unnecessary antibiotic prescribing.

What the next page fixes

Chapter 17.3 covers the remaining categories — fungi, which are harder to treat because they are our relatives, and parasites, which infect over a billion people and are among the most treatable conditions in global health.