Contagious for four days before the rash appears

Measles is an acute viral respiratory illness, and **one of the most
contagious diseases there is**. The **MMR vaccine provides the best
protection**.

The timing problem

**Patients are contagious from 4 days before the rash appears to 4 days
after.**

The rash is the sign that makes measles recognisable — and it arrives four
days into the infectious period. For most of the time a person is spreading
measles, neither they nor anyone around them has any reason to suspect it.

And sometimes immunocompromised patients never develop the rash at all.

The rash itself usually appears about 14 days after exposure, and spreads
head → trunk → lower extremities.

What it looks like before that

A prodrome of:

• Fever, as high as 105°F
• Malaise
• The three C's — cough, coryza (runny nose), conjunctivitis
• Koplik spots — a pathognomonic enanthema, meaning spots inside the mouth
  that occur in measles and essentially nothing else

Koplik spots are the diagnostic gift here: they appear before the rash, and
finding them can identify measles during the window when it is spreading
invisibly.

What clinicians are asked to do

• Isolate infected patients for 4 days after the rash develops, and follow
  airborne precautions in healthcare settings
• Report suspected measles cases to your local health department
• Laboratory confirmation is essential for all sporadic cases and all
  outbreaks

That third point matters more than it sounds. A clinical diagnosis is enough
to isolate a patient and start a response; it is not enough to establish that
an outbreak is measles, or to link cases to each other.

*Continuing education is available: "Clinical Overview of Measles: Diagnosis,
Laboratory Testing, and Outbreak Response" (WD4728) on CDC TRAIN, requiring a
post-assessment score of 80%.*

Source: Centers for Disease Control and Prevention.