Allergic Reaction: This is often the most common cause of conjunctivitis and rash, especially if the symptoms are acute and there's a clear history of exposure to an allergen. Allergic reactions can range from mild to severe and can be caused by a variety of substances including pollen, dust, pet dander, and certain chemicals or drugs.

Viral Exanthem: Many viral infections can cause both conjunctivitis and rash. Common examples include measles, rubella, and adenovirus infections. The presence of fever, cough, or other systemic symptoms can help differentiate these conditions.
Bacterial Conjunctivitis with Secondary Rash: Certain bacteria like Staphylococcus aureus or Streptococcus pneumoniae can cause conjunctivitis, and in some cases, a secondary rash may develop, either due to the infection itself or as an allergic reaction to the bacteria or its toxins.
Eczema (Atopic Dermatitis): Eczema can sometimes cause eye symptoms including conjunctivitis, especially if the eczema involves the face or eyelids. A rash is a hallmark of eczema, making it a plausible diagnosis when both symptoms are present.

Kawasaki Disease: Although rare, Kawasaki disease is a critical diagnosis not to miss because of its potential to cause coronary artery aneurysms. It presents with fever, rash, conjunctivitis (usually bilateral and non-exudative), and other symptoms like strawberry tongue and cervical lymphadenopathy.
Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN): These are severe skin and mucous membrane disorders usually caused by medications or infections. They can present with conjunctivitis and rash, among other symptoms, and are medical emergencies due to their high morbidity and mortality.
Meningococcemia: This is an infection caused by Neisseria meningitidis and can present with a rash and conjunctivitis, along with severe systemic symptoms like fever and meningitis. It's a medical emergency due to its rapid progression and high mortality rate if not promptly treated.

Relapsing Fever: Caused by Borrelia species, this condition can present with recurring episodes of fever, rash, and conjunctivitis, among other symptoms. It's transmitted by ticks or lice and is more common in certain geographic areas.
Leptospirosis: A zoonotic infection caused by Leptospira bacteria, which can cause a wide range of symptoms including conjunctivitis and rash, in addition to fever, headache, and muscle aches. It's more common in individuals who work with animals or are exposed to contaminated water.
Syphilis: In its secondary stage, syphilis can cause a variety of symptoms including rash and conjunctivitis. Although less common, it's an important diagnosis to consider, especially in sexually active individuals or those with a history of sexually transmitted infections.