Influenza-like illness

Overview

Influenza-like illness (ILI) is a clinical term used to describe a group of symptoms that resemble those caused by influenza (the flu) but may be due to a variety of viral or bacterial pathogens. ILI is often characterized by the sudden onset of fever, chills, cough, sore throat, and muscle aches. It is commonly used in surveillance systems to track flu activity in the population, especially when laboratory confirmation is not available. ILI is not a specific diagnosis but rather a symptom-based definition used for early detection and public health monitoring.

Causes

While true influenza viruses (influenza A and B) are common causes of ILI, many other pathogens can produce similar symptoms. These include:

• Respiratory syncytial virus (RSV)

• Human rhinovirus (cause of the common cold)

• Parainfluenza viruses

• Coronavirus (including SARS-CoV-2)

• Adenovirus

• Human metapneumovirus

• Mycoplasma pneumoniae or Chlamydophila pneumoniae (atypical bacterial causes)

ILI is typically spread through respiratory droplets, direct contact with infected individuals, or touching contaminated surfaces.

Symptoms

Symptoms of influenza-like illness usually begin abruptly and may include:

• Fever (usually ≥ 100°F or 37.8°C)

• Chills and sweats

• Cough (usually dry)

• Sore throat

• Runny or stuffy nose

• Muscle aches (myalgia)

• Headache

• Fatigue and weakness

• Occasional gastrointestinal symptoms (such as nausea, vomiting, or diarrhea, more common in children)

Diagnosis

Diagnosis of ILI is primarily clinical, especially during flu season or outbreaks. However, when precise identification is needed, the following may be used:

• Medical history and symptom assessment: Onset, duration, and combination of symptoms help guide clinical judgment.

• Rapid influenza diagnostic tests (RIDTs): Can provide quick results but may have limited sensitivity.

• PCR testing: A more accurate method for detecting specific viruses, including influenza and SARS-CoV-2.

• Chest X-ray: Performed if pneumonia or another complication is suspected.

• Blood tests: Rarely necessary unless sepsis or another serious condition is suspected.

Treatment

Treatment of ILI depends on the underlying cause and severity of symptoms. In most cases, supportive care is sufficient:

• Rest and hydration: Adequate sleep and fluid intake help the body recover.

• Antipyretics and analgesics: Medications like acetaminophen or ibuprofen relieve fever, headache, and muscle aches.

• Antiviral medications: Drugs such as oseltamivir (Tamiflu) may be prescribed for confirmed or suspected influenza, especially in high-risk groups.

• Isolation and infection control: To prevent transmission, affected individuals should stay home and avoid close contact with others.

• Antibiotics: Only used if a secondary bacterial infection is confirmed or strongly suspected.

Prognosis

The prognosis for influenza-like illness is generally good, especially in healthy individuals. Most people recover within a few days to a week without complications. However, severe illness and complications such as pneumonia, bronchitis, or worsening of chronic conditions (e.g., asthma, heart disease) can occur, particularly in:

• Infants and young children

• Elderly individuals

• Pregnant women

• People with chronic medical conditions or weakened immune systems

Preventive measures, including annual influenza vaccination, good hand hygiene, and respiratory etiquette, play a critical role in reducing the incidence and spread of ILI.