13.2 Common Viral & Bacterial Primary Care Infections

Most infectious-disease volume in primary care is not Kawasaki and not measles. It is the toddler who defervesced into a rash, the slapped-cheek schoolchild whose teacher is pregnant, the bag urine that is not a diagnosis, and the caregiver who wants amoxicillin for three days of green snot. Domain II is the pattern. Domain III is when antibiotics are not indicated and when they are. Community pneumonia is introduced here; depth of respiratory examination and wheeze lives in the pulmonology chapter.

Clinic opening. An 11-month-old had three days of high fever, looked surprisingly well, then woke up afebrile with a pink maculopapular trunk rash. If you start Kawasaki workup on hour one of the rash, or if you give amoxicillin "because fever plus rash might be strep," you missed roseola.

Viral illnesses you must name by timing

Roseola (exanthem subitum, HHV-6, sometimes HHV-7) typically hits 6–24 months. High fever for about 3–5 days in a child who is often playful or only mildly ill, then defervescence and a rose-pink maculopapular rash on the trunk that spreads. Febrile seizure can occur with the fever, not because the rash is dangerous. The rash is not contagious in the measles sense. You counsel that the illness is ending, not beginning. Trap: calling roseola measles, or calling it Kawasaki on the first day the rash appears after the fever has already broken.

Fifth disease (erythema infectiosum, parvovirus B19) is a school-age slapped-cheek appearance, then a lacy, reticular rash on the trunk and extremities that can recede and flare with heat or sun. The child is contagious before the rash, during the viral prodrome. Once the slapped-cheek rash is present, the child is not considered contagious and can usually return to school if otherwise well. Pregnancy exposure counseling is mandatory: parvovirus can cause fetal anemia and hydrops. Identify pregnant contacts, advise them to call obstetrics, and do not panic-order titers on the child as if that replaced OB care. In children with hemolytic disorders, B19 can trigger aplastic crisis — that is an ED problem, not a cute rash.

Hand-foot-and-mouth disease (coxsackie A and other enteroviruses) is fever plus painful oral vesicles/ulcers and vesicles on palms, soles, and often buttocks. Coxsackie A6 can make a more extensive, peeling eruption. Care is supportive: hydration, pain control, watch for decreased urine. Nail shedding weeks later is a known aftermath, not a new emergency fungus. Herpangina is enteroviral vesicles on the posterior oral mucosa without the hand-foot findings.

HSV gingivostomatitis (usually HSV-1) is anterior mouth disease: gingiva, lips, tongue, fever, drooling, refusal to drink. The safety check is hydration. Oral acyclovir or valacyclovir can help when started early in selected primary cases; the child who cannot drink goes to the ED. Herpetic whitlow is a finger vesicle after autoinoculation. Do not call anterior herpetic gingivostomatitis "just HFMD" when the gums are swollen and the child is toxic with drool.

EBV (infectious mononucleosis) is fatigue, fever, exudative pharyngitis, cervical lymphadenopathy, and often splenomegaly in an adolescent or older child. Rapid strep may be negative; heterophile or EBV serology supports the diagnosis when you need it. Do not give amoxicillin or ampicillin for EBV. The combination commonly produces a diffuse morbilliform rash. That rash is not a penicillin allergy for life until proven otherwise, but it is an exam trap: amoxicillin causes rash in EBV. Supportive care, activity limited while the spleen may be enlarged (avoid contact sports until the spleen is not enlarged on exam — often several weeks), and airway watch if tonsils are kissing. CMV can mimic a mono-like illness (fever, malaise, hepatitis, often less pharyngitis than EBV) in immunocompetent children; congenital CMV is a different conversation. You do not need a CMV PCR on every tired teenager; you do need to remember CMV exists when EBV testing is negative and the story fits.

Bacterial problems the office actually treats

Community pneumonia (approach, not the full pulmonary chapter). Viral pneumonia is common in preschoolers. Typical bacterial CAP in an immunized, well-oxygenated child who can drink is often treated with high-dose amoxicillin aimed at pneumococcus. School-age walking pneumonia (Mycoplasma) is the atypical pattern: gradual cough, lowish fever, not toxic — a macrolide is the usual primary-care choice when that pattern is clear. Distress, hypoxia, infant age, dehydration, or suspected round pneumonia/effusion leave the office. Wheeze, bronchiolitis, and asthma-versus-pneumonia depth belong in pulmonology. Do not add a "just in case" third antibiotic because the cough has lasted four viral days.

Cellulitis and MSSA versus MRSA skin. Nonpurulent cellulitis (expanding warmth, no drainable pus) is often streptococcal — cephalexin or another beta-lactam covering MSSA/strep is the usual first oral agent in a well child. Purulent infection, abscess, or known MRSA contacts shifts you toward MRSA-active oral therapy (typically trimethoprim-sulfamethoxazole or clindamycin, by local resistance) plus drainage when there is a fluctuant collection. Mark the border. Recheck. Rapidly spreading, orbital, or toxic disease is emergency care. Do not treat a mature abscess with antibiotics alone (see dermatology for I&D).

UTI diagnosis (the collection rules again, because this is where they are missed). Leukocyte esterase and pyuria support the diagnosis; culture is the gold standard. Catheter or SPA for infants who are not toilet trained when you need a culture. Bag urine is a screen: negative helps; positive does not finish the case. Treat the young febrile child as pyelonephritis. Asymptomatic bacteriuria in a well toilet-trained child is not an automatic antibiotic course.

Scarlet fever is group A strep pharyngitis plus a sandpaper rash, flushed cheeks, perioral pallor, and strawberry tongue. Confirm strep. Treat with penicillin or amoxicillin. Return to daycare after 24 hours of antibiotics if afebrile and improving. Sandpaper rash without testing is how you both miss Kawasaki and overcall viral exanthems as scarlet fever.

Lyme disease. In an endemic area, classic erythema migrans — expanding erythematous patch, often annular with central clearing, usually not very itchy — is a clinical diagnosis. Treat without serology. Early serology is often negative and delays doxycycline (AAP/Red Book: a course of 21 days or fewer is acceptable even under age 8 for this indication). Flu-like symptoms can accompany EM. Late Lyme (arthritis, neuropathy) is a serology-plus-referral problem. A circular "spider bite" that grows over days in July in New England is EM until you have a better story.

When antibiotics are not indicated

This is as high-yield as any drug name.

Do not treat bronchiolitis with a Z-Pak. Do not treat 5 days of viral nasal congestion as sinusitis. Do not treat every exudative throat with amoxicillin before a strep test — that is how you manufacture the EBV rash trap.

Return to daycare and school

AAP exclusion logic is can the child participate, is there fever, and is there a specific communicable period — not a universal day count. Strep and many bacterial skin infections: typically 24 hours of effective antibiotics and improving. Fifth disease: may attend once the rash is present if well. Roseola: return when afebrile and well. HFMD: many settings exclude while febrile; lesions need not be fully healed if the child can participate — follow local public health, do not invent a national crusting rule. Viral URI: exclude for fever or inability to participate, not for residual clear runny nose. Do not recycle a stale COVID isolation day count as a daycare policy for these illnesses (see 13.3).

Clinic vignette. A 7-year-old has slapped cheeks and a lacy arm rash, is afebrile, and wants to go to school. Teacher is 8 weeks pregnant. Plan: the child can usually attend; the child was contagious before the rash; the teacher needs obstetric counseling, not a lecture that the child must stay home 10 more days.

Clinic close. Fever then rash in a well toddler: roseola. Slapped cheek: B19, contagious already finished, pregnancy counseling. Anterior mouth plus drool: HSV, hydrate. Exudative pharyngitis in a teen: think EBV, hold the amoxicillin. Bag UA is not a UTI. Classic bull's-eye: treat Lyme. Green snot on day 4: not an antibiotic. That is this section.