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Prepositions after adjectives

Infected with, by, in, from or at?

Infected with is the standard pattern for diseases, viruses, or harmful software. Infected by can name the source or agent.

Quick answer

Which preposition follows infected?

Use infected with for the illness, virus, or harmful thing itself. Use infected by for the source or agent causing the infection. Infected in, infected from, and infected at are usually not standard directly after the adjective, although from or at may appear separately with other meanings in a sentence.

She was infected with the virus.
The files were infected by malware.
Several plants became infected with fungus.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
infected with + disease/virus

Naming the infection

Use with for the disease, virus, bacteria, or harmful software.

He was infected with malaria.
The computer is infected with spyware.
2
infected by + source

Naming the cause or agent

Use by to identify what caused the infection.

The crops were infected by contaminated water.
Several users were infected by a phishing attack.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
infected with
She was infected with influenza.
Standard pattern for the illness or harmful substance.
infected by
The network was infected by a virus.
Used for the agent or source causing infection.
infected in
He was infected in hospital.
In can introduce place, not the complement of infected.
infected from
She was infected from contaminated food.
Possible in some contexts to indicate source, but less common than by or through.
infected at
He was infected at school.
At introduces location, not the infection itself.

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using in for the illness

Use with to name the disease or virus.

✕ She was infected in flu. ✓ She was infected with flu.

Using at directly after infected

At usually introduces a place, not the cause of infection.

✕ The computer was infected at malware. ✓ The computer was infected with malware.

Quick practice

Choose the expression that matches the intended meaning.

1. The patient was infected ___ dengue fever.
2. The system was infected ___ a malicious email attachment.
3. He was infected ___ hospital during the outbreak.

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Frequently asked questions

What is the difference between infected with and infected by?

With names the infection itself, while by names the source or cause.

Can infected from be correct?

Sometimes. It may describe the source of infection, but infected by or infected through is usually more natural.

Can I say infected at school?

Yes. Here at school describes the location where the infection happened.

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