infectious

infectious — adjective

1. describes a sickness that can travel from one living thing to another through th

1.adjectiveB1
Definition

describes a sickness that can travel from one living thing to another through the air, water, or touch

Example

The flu is highly infectious and spread through the school within a week.

collocation: highly infectious

Doctors warned that the virus remained infectious on metal surfaces for up to three days.

remain + infectious (linking verb pattern)

Common collocations
  • highly infectious
  • infectious disease
  • infectious illness
  • infectious virus
  • infectious strain
  • remain infectious
  • become infectious
Synonyms
  • contagious

    more specific — describes diseases spread through direct touch or contact, not air or water

  • communicable

    more formal — used in official health notices and medical writing

  • transmissible

    technical — used for diseases that can be passed between organisms, often through blood or fluids

Antonyms
  • non-infectious

    describes a disease that cannot spread to others

  • non-communicable

    formal term for diseases like diabetes or heart disease that cannot be passed between people

Grammar Patterns

infectious + noun (disease, illness, virus)

be / remain / become infectious

Usage Note

Commonly placed before a noun to name the disease type (infectious disease, infectious virus). Distinguish from contagious: contagious describes diseases passed specifically through touch or close contact, while infectious can also include airborne or waterborne spread.

Common Mistakes

I caught an infectious cold from shaking hands with my colleague.
I caught a contagious cold from shaking hands with my colleague.
Colds spread through touch, so contagious is more precise for contact-based transmission.
The doctor said my wound was infectious.
The doctor said my wound was infected.
A wound that has the disease inside it is infected; infectious means it can spread to others.

2. describes a person or animal who has a sickness that could spread to other peopl

2.adjectiveB1
Definition

describes a person or animal who has a sickness that could spread to other people, animals, or plants around them

Example

Tara was still infectious after her fever faded, so she kept wearing a mask.

be + still/no longer + infectious (state of person)

The vet said the puppy was infectious and needed to stay away from other dogs.

Common collocations
  • still infectious
  • no longer infectious
  • remain infectious
  • become infectious
  • highly infectious (person)
Synonyms
  • contagious

    same meaning for people carrying a disease, but slightly more common in everyday speech

  • infected

    focuses on having the disease inside the body, not necessarily on the ability to spread it

  • carrying

    informal — 'carrying a virus' suggests the person may not show symptoms

Antonyms
  • immune

    protected against a disease and unable to catch or spread it

  • recovered

    no longer has the disease and therefore cannot spread it

Grammar Patterns

be infectious

remain infectious

become infectious

no longer infectious

Usage Note

Almost always used after a linking verb (be, remain, become, feel) rather than before a noun. Unlike sense 1, you would not say 'an infectious person' to mean a person carrying disease — instead say 'a person who is infectious' or 'an infected person.'

Common Mistakes

The doctor said I am an infectious patient.
The doctor said I am still infectious.
For people, infectious follows a linking verb; placing it before a noun sounds unnatural in medical contexts.

3. describes an emotion, quality, or activity that quickly spreads among a group of

3.adjectiveB2
Definition

describes an emotion, quality, or activity that quickly spreads among a group of people and makes them feel the same way or want to take part

Example

Manuela's infectious laughter filled the room and soon everyone was smiling.

infectious + laughter (most common figurative collocation)

The coach's infectious enthusiasm made the whole team train harder than ever.

Common collocations
  • infectious laughter
  • infectious enthusiasm
  • infectious energy
  • infectious joy
  • infectious smile
  • infectious excitement
Synonyms
  • contagious

    works figuratively too (contagious laughter), but sounds slightly more playful or less formal

  • catching

    informal — 'His good mood was catching' is friendly and natural in speech

  • compelling

    stronger — suggests a force that makes people feel they must follow or agree

Antonyms
  • dull

    describes a mood or atmosphere that does not excite or spread

  • unexciting

    describes something that fails to spark interest in others

Grammar Patterns

infectious + noun (laughter, enthusiasm, joy, energy, excitement)

be infectious

Usage Note

This is the figurative (metaphorical) sense — feelings, not germs, are doing the spreading. Most common with nouns for positive emotions: laughter, enthusiasm, joy, excitement, energy. Cannot be used for negative emotions (sadness, anger) — those are described as contagious in informal use, but not infectious.

Common Mistakes

Her sadness was infectious and made everyone cry.
Her sadness spread through the room and made everyone cry.
Infectious is rarely used for negative emotions in standard English.

Word family

Synonym discussion

Sense: adjective/3

All four words describe something that spreads from person to person. Infectious is the most neutral and common choice for positive feelings that naturally influence others — it works in both formal and everyday contexts (infectious laughter, infectious enthusiasm). Contagious has the same figurative use but carries a slightly playful or informal tone because its primary meaning relates to physical illness; saying 'her smile was contagious' feels more vivid and slightly humorous than 'her smile was infectious.' Catching is fully informal and used mostly in spoken English ('his good mood was catching' — natural among friends but too casual for writing). Compelling is the strongest of the four — it implies not just spread but a force that makes people feel they must respond, so it fits persuasive speeches or powerful arguments better than spontaneous joy. For example: 'The speaker's compelling argument convinced the whole audience to vote' (strong push to action) versus 'The speaker's infectious energy made the audience smile' (natural emotional spread, no obligation).

infectious ↗contagious ↗catching ↗compelling ↗

Common collocations

Adverbs

Phrases

  • become infectious
  • remain infectious

Etymology

LatinEnglish

The word infectious comes from the Latin infectiosus, meaning 'infected or contaminated,' which developed from inficere — 'to put in, stain, or taint.' The prefix in- means 'in' and facere means 'to do or make.' The word was first used for dyeing cloth, then shifted to disease in the 1500s. By the 1600s, English speakers were already using the word figuratively for things like laughter and enthusiasm that 'spread' among people like a disease.