Consequently, the date a rash appears cannot reliably establish when an infection was acquired. A symptom developing weeks after sexual contact does not prove that the contact caused it. Likewise, the absence of symptoms immediately after exposure does not prove that transmission did not occur.
Other explanations for widespread red patches
Not every alarming skin condition is infectious. Psoriasis, for example, can cause red plaques covered with scales, particularly on the knees, elbows, scalp, and other areas. Some people develop extensive inflammation, while others experience only small patches.
Eczema can produce redness, dryness, itching, cracking, and inflammation. Allergic contact dermatitis may develop after exposure to detergents, fabrics, cosmetics, adhesives, or other substances. Certain medicines can cause widespread rashes, sometimes requiring urgent medical attention.
Circulation-related disorders can also alter the appearance of the lower legs. Some cause reddish or brownish discoloration, swelling, or changes in skin texture. Other conditions may produce small red or purple spots because of bleeding beneath the skin.
These possibilities matter because treatments differ substantially. A cream appropriate for eczema may be inadequate for an infection, while unnecessary antibiotics can cause side effects and contribute to antimicrobial resistance.