Final answer:
Theo's symptoms suggest he might be suffering from Fifth disease, caused by parvovirus B19. With modern medical care, the disease is not typically fatal, unlike in 1900. The rash is an immune response to the virus, and the treatment likely involves fever reducers with diarrhea as a possible but unrelated symptom.
Step-by-step explanation:
Possible Diagnosis and Pathogen
The symptoms described for Theo (fever, lethargy, sore throat, vomiting, stomach pain, rash) mimic those associated with Fifth disease, also known as erythema infectiosum, which is caused by parvovirus B19. Theo's presentation with flushed cheeks and a pink rash on his body are characteristic of this illness. Considerations for a different diagnosis would be needed if he did not present with the rash.
Pathogen Description
Parvovirus B19 is the pathogen behind Fifth disease, spread through respiratory secretions from an infected individual. The disease mostly affects children and presents with a 'slapped cheek' appearance followed by a spreading rash.
Risk of Fatality
In contrast to 1900, today's medical advancements and antibiotics make it unlikely for Theo's condition to be fatal. In an earlier era without these treatments and less knowledge of disease management, the risk would have been higher.
Causes of the Rash
The rash from Fifth disease is due to the immune system's response to the infection from parvovirus B19.
Alternate Scenario Without Rash
Without the presence of a rash, the diagnosis could be challenging and may require the consideration of other illnesses such as strep throat or roseola, depending upon the full spectrum of symptoms.
Treatment Options
The doctor may prescribe supportive care for Theo, including fever reducers such as acetaminophen or ibuprofen. Due to the viral nature of the disease, antibiotics would not prove beneficial.
Side Effects of Treatment
Diarrhea is not a typical side effect of treatment with fever reducers, indicating that it could be either a continuation of the initial gastro-intestinal symptoms of Fifth disease or due to another underlying cause.