Measles

Measles is the most contagious human virus known. It spreads through the air and lingers in a room for up to two hours after the infected person has left — no direct contact required. In a fully susceptible population, roughly 90% of close contacts will be infected.

What it looks like

Seven to fourteen days after exposure: high fever (often above 104°F), cough, runny nose, and red watery eyes. Two to three days in, tiny white spots (Koplik spots) may appear inside the mouth. The rash follows, starting at the hairline and moving down the body. A person is contagious from about four days before the rash appears until four days after — meaning transmission largely happens before anyone knows it's measles.

Two things that are underappreciated

• Immune amnesia. Measles infects and destroys memory immune cells, erasing part of the immunity you had built to other pathogens. Studies have measured the loss of 11% to 73% of pre-existing antibody repertoire. The practical consequence is a period of two to three years after recovery with elevated susceptibility to other infections, which shows up in population data as excess non-measles childhood mortality following measles epidemics.
• SSPE. Subacute sclerosing panencephalitis is a fatal degenerative brain disease that appears 7 to 10 years after measles infection. There is no treatment. Risk is highest for children infected under age 2 — the same children who are too young to be vaccinated and who depend on everyone around them being immune.

Whooping cough (pertussis)

A bacterial infection that is a miserable multi-week illness in adults and a life-threatening one in infants. About one third of infants under 12 months who get pertussis are hospitalized; among those hospitalized, roughly 1 in 100 dies. Nearly all pertussis deaths are in babies under 3 months — too young to be well protected by their own vaccine doses.

The characteristic "whoop" is the sound of a desperate breath in after a coughing fit. Infants often don't whoop at all; instead they may simply stop breathing (apnea), which is why an infant with any cough deserves close attention.

It is often called the "100-day cough" because the coughing phase routinely lasts 10 weeks or more. Antibiotics shorten how long you're contagious and help if started very early, but they do little for the cough once the paroxysmal phase begins — the damage to the airway lining is already done.

The most effective protection for a newborn is Tdap during pregnancy at 27–36 weeks, which transfers antibodies across the placenta and is roughly 78–91% effective at preventing pertussis in infants under 2 months. Everyone who will be around a newborn should also be up to date — this "cocooning" strategy exists because the source of an infant's infection is usually a family member with a cough they assumed was nothing.

Influenza

Seasonal flu causes on the order of tens of thousands of U.S. deaths in a typical year, concentrated in adults over 65, though a meaningful number of pediatric deaths occur each season — and the majority of those children were unvaccinated.

Flu is not a bad cold. The distinguishing features are abrupt onset (people often name the hour they felt it start), high fever, and body aches severe enough to be the dominant symptom. Colds arrive gradually and center on the nose and throat.

Antivirals

Oseltamivir and baloxavir shorten illness by roughly a day in otherwise healthy people — a modest benefit. The case is much stronger for people at high risk of complications (over 65, pregnant, chronic lung or heart disease, immunocompromised, under 2), where the goal is preventing hospitalization rather than saving a day of symptoms. They work best started within 48 hours of onset, so a same-day call matters.

Why the vaccine is less effective than most

Influenza mutates rapidly, and each year's vaccine must be formulated months ahead based on which strains are predicted to circulate. When the match is good, effectiveness against symptomatic illness is roughly 40–60%; in a mismatched year it's lower. This is a genuine limitation and worth stating plainly. It's also why the vaccine remains worthwhile: even partial protection substantially reduces hospitalization and death, and vaccinated people who get infected tend to have milder illness and shorter contagious periods.

COVID-19

SARS-CoV-2 has settled into a pattern of recurring waves rather than disappearing. It is no longer a novel pathogen for most people — nearly everyone has some immunity from vaccination, infection, or both — but it remains a significant cause of severe illness in older adults and immunocompromised people.

Risk is very unevenly distributed

The risk gradient with age is one of the steepest observed for any infectious disease: severe outcomes rise roughly exponentially from middle age onward. Other major risk factors include immunosuppression, chronic kidney disease, obesity, diabetes, and chronic lung disease. A healthy 25-year-old and an 80-year-old with kidney disease are not facing the same illness, and guidance that treats them identically is being imprecise in a way that costs credibility.

Long COVID

Persistent symptoms after infection — fatigue, cognitive difficulty, breathlessness, autonomic dysfunction — are real and measurable, not psychosomatic. Prevalence estimates vary widely (from a few percent to over ten percent of infections) because case definitions and study designs differ so much; that spread reflects genuine methodological difficulty rather than a cover-up in either direction. What the evidence does support reasonably consistently: risk is lower after vaccination, lower with subsequent infections than first ones, and higher after severe acute illness.

Treatment

Nirmatrelvir-ritonavir (Paxlovid) reduces hospitalization substantially in high-risk patients when started within five days of symptom onset. Benefit in low-risk, vaccinated, otherwise healthy adults is much smaller and not clearly established. It interacts with a long list of common medications, so it needs a real prescriber review rather than a checkbox.

RSV (respiratory syncytial virus)

Nearly every child is infected with RSV by age 2. For most it's a cold. For infants — especially those under 6 months, born prematurely, or with heart or lung conditions — it can cause bronchiolitis, inflammation of the small airways that makes breathing genuinely hard work. RSV is the leading cause of hospitalization in U.S. infants.

It's also underrecognized in older adults, where it causes tens of thousands of hospitalizations a year, with severity comparable to influenza.

What to watch for in an infant: fast breathing, the skin pulling in between or below the ribs with each breath, nostril flaring, grunting, pauses in breathing, or feeding much less than usual. Wheezing that a parent describes as "working hard to breathe" is the phrase clinicians take most seriously.

Prevention changed substantially with two tools that arrived recently: nirsevimab, a long-acting antibody given to infants, and a maternal RSV vaccine at 32–36 weeks of pregnancy. Both markedly reduce infant RSV hospitalization. One or the other is used — generally not both.

HPV and cancer

Human papillomavirus is so common that most sexually active people acquire it at some point. The great majority of infections clear on their own. Persistent infection with a high-risk type is what causes cancer — and it causes essentially 100% of cervical cancer, plus most anal, oropharyngeal, vaginal, vulvar, and penile cancers.

This makes HPV vaccination one of the few genuinely cancer-preventing vaccines available. The effect size is unusually large: a Swedish study of nearly 1.7 million people found substantially reduced invasive cervical cancer incidence among vaccinated women, with the largest reduction among those vaccinated before age 17. Scotland has reported no cases of invasive cervical cancer among women vaccinated at ages 12–13.

The vaccine works by preventing infection, so it is dramatically more effective given before exposure — which is why it's recommended at 11–12, an age that has nothing to do with anticipating a child's behavior and everything to do with immunology. Antibody response is also strongest at that age, which is why two doses suffice before 15 and three are needed after.

Meningococcal disease

Rare, but included here because it is fast and because recognizing it early genuinely changes outcomes. Neisseria meningitidis can go from vague flu-like symptoms to death in under 24 hours. Even with prompt antibiotics, roughly 10–15% of those infected die, and 10–20% of survivors are left with permanent consequences — deafness, brain damage, or limb amputation from septic shock.

The signs that matter: fever with severe headache and a stiff neck; confusion; sensitivity to light; and a rash of small red or purple spots that does not fade when pressed with a clear glass. That non-blanching rash is a late sign and a medical emergency — do not wait for it to appear before seeking care, and do not wait to see whether it develops before going in.

Risk is elevated in adolescents and young adults, particularly in congregate living like college dorms and military barracks — which is why the vaccine is scheduled at 11–12 with a booster at 16, timed to cover the years of highest risk.

Polio

Most polio infections cause no symptoms at all, which is precisely what makes it dangerous: the virus circulates silently. In roughly 1 in 200 infections it invades the nervous system and causes irreversible paralysis; of those paralyzed, 5–10% die when breathing muscles are affected.

Polio is close to eradication globally but is not gone. Wild poliovirus still circulates in a small number of countries, and vaccine-derived poliovirus can emerge and spread in under-vaccinated communities. A 2022 case of paralytic polio in an unvaccinated adult in New York — with the virus subsequently detected in wastewater across several counties — is the clearest recent demonstration that low local coverage re-opens the door.

The U.S. uses inactivated polio vaccine (IPV), which contains no live virus and cannot cause vaccine-derived polio.

Antibiotics: when they help and when they don't

Antibiotics kill bacteria. They do nothing to viruses. Colds, flu, COVID-19, RSV, and most sore throats, bronchitis, and sinus infections are viral. Taking an antibiotic for these provides no benefit while carrying real costs: 1 in 5 antibiotic prescriptions results in a side effect, and C. difficile colitis — an infection caused by wiping out protective gut bacteria — kills thousands of Americans annually.

Antimicrobial resistance is not a distant hypothetical. Resistant infections already cause well over a million deaths worldwide each year, and the pipeline for genuinely new antibiotic classes is thin. Every unnecessary course selects for resistant bacteria in your own body and in the community.

Two practical rules: if you're prescribed antibiotics, take the full course as directed rather than stopping when you feel better; and never take leftover antibiotics, someone else's prescription, or antibiotics bought without one — wrong drug, wrong dose, and wrong duration all drive resistance while treating nothing.

Reducing your risk, ranked by how much it does

1. Vaccination. By a wide margin the highest-leverage action for the diseases on this page.
2. Stay home when sick. The single most effective behavioral measure, and the one most often skipped. Most respiratory transmission happens in the first days of symptoms.
3. Ventilation. Respiratory viruses accumulate in indoor air. Opening windows, running HVAC on fresh air, using a portable HEPA filter, or moving a gathering outdoors reduces exposure substantially — for a fraction of the effort of most other measures.
4. Hand hygiene. Very effective for gastrointestinal illness (norovirus, rotavirus) and for viruses like RSV that transmit well on surfaces. Less decisive for airborne spread than it was once presented as being. Soap and water beat sanitizer against norovirus and C. difficile.
5. Masks in high-risk settings. Well-fitted respirators (N95/KN95) meaningfully reduce inhaled exposure; fit matters more than material. Most useful for a specific reason at a specific time — during a local surge, in crowded indoor spaces while immunocompromised, or when caring for someone sick.
6. Prompt treatment. For flu and COVID-19, antivirals have a narrow window. If you're at higher risk, test early and call early rather than waiting to see how it goes.

Last reviewed: August 2026. Complication rates cited here are drawn from published surveillance data and peer-reviewed literature; see Evidence & Sources. Figures are approximate and vary by population, era, and access to care.