Viruses That Spread From Person to Person

Knowing how a virus travels tells you what to do about it. A mask is highly relevant for measles and largely irrelevant for hepatitis B. Handwashing matters enormously for norovirus and very little for chickenpox. The precaution has to match the route.

The categories below overlap at the edges: several viruses use more than one route, but they are the practical framework clinicians use, and they explain why outbreak advice differs so much between infections.

Airborne transmission

The most efficient route. The virus survives in tiny aerosol particles that stay suspended in air for hours and travel well beyond conversational distance. Measles is the classic example: it can infect someone who enters a room up to two hours after an infected person has left.

This is why measles requires such high population vaccination coverage to prevent outbreaks, around 95 percent, where less transmissible infections need considerably less.

• Measles: extraordinarily contagious; one case typically infects 12 to 18 unprotected people.
• Chickenpox (varicella): airborne plus direct contact with the rash fluid.
• Tuberculosis: bacterial rather than viral, but airborne and highly relevant in India.

Droplet transmission

Larger respiratory droplets that fall out of the air within a metre or two. Requires closer contact than airborne spread, which is why distance and masks work well against these.

• Influenza: droplets plus contaminated surfaces.
• SARS-CoV-2: droplet and aerosol, particularly in poorly ventilated indoor spaces.
• Mumps and rubella: droplets from coughing, sneezing and talking.
• Pertussis (whooping cough): bacterial, highly transmissible in households.
• RSV: droplets and contaminated hands and surfaces.

Faecal-oral transmission

Spread through contaminated water, food washed in it, or food handled without proper hand hygiene. This is the dominant route for several infections that remain common in India, and it is closely tied to sanitation and water safety rather than to interpersonal contact.

• Hepatitis A: vaccine-preventable.
• Hepatitis E: waterborne, particularly dangerous in pregnancy, vaccine not widely available.
• Poliovirus: vaccine-preventable; the reason polio eradication depends on sanitation as well as vaccination.
• Rotavirus and norovirus: major causes of gastroenteritis; rotavirus is vaccine-preventable.
• Typhoid: bacterial, vaccine-preventable.

Blood-borne and sexual transmission

These require direct exchange of blood or body fluids. They do not spread through casual contact, and the stigma attached to them often exceeds the actual transmission risk in everyday settings.

• Hepatitis B: vaccine-preventable; also transmitted mother to child at birth.
• Hepatitis C: no vaccine, but curable with antiviral treatment.
• HIV: no vaccine; prevented with barrier methods, PrEP and safe injecting practice.
• HPV: vaccine-preventable; the cause of most cervical cancer.

Vector-borne infections are a different category

Dengue, chikungunya, malaria and Japanese encephalitis are not transmitted person to person at all. They need an intermediate host, usually a mosquito, to carry them between people.

This matters practically: isolating a dengue patient achieves nothing for transmission, whereas mosquito control and bite avoidance achieve a great deal. Japanese encephalitis is vaccine-preventable; dengue and chikungunya are not, in routine practice here.

Why some people spread infection without symptoms

Several viruses are transmissible before symptoms appear or without ever producing them. Measles is contagious for about four days before the rash. Hepatitis B can be carried silently for decades. Polio infection is asymptomatic in the large majority of cases.

This is the core argument for vaccination as a community measure rather than a purely individual one. You cannot reliably avoid infected people, because they frequently do not know they are infected.

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