COVID-19 is a pandemic viral disease with catastrophic global impact. This 
disease is more contagious than influenza such that cluster outbreaks occur 
frequently. If patients with symptoms quickly underwent testing and contact 
tracing, these outbreaks could be contained. Unfortunately, COVID-19 patients 
have symptoms similar to other common illnesses. Here, we hypothesize the order 
of symptom occurrence could help patients and medical professionals more quickly 
distinguish COVID-19 from other respiratory diseases, yet such essential 
information is largely unavailable. To this end, we apply a Markov Process to a 
graded partially ordered set based on clinical observations of COVID-19 cases to 
ascertain the most likely order of discernible symptoms (i.e., fever, cough, 
nausea/vomiting, and diarrhea) in COVID-19 patients. We then compared the 
progression of these symptoms in COVID-19 to other respiratory diseases, such as 
influenza, SARS, and MERS, to observe if the diseases present differently. Our 
model predicts that influenza initiates with cough, whereas COVID-19 like other 
coronavirus-related diseases initiates with fever. However, COVID-19 differs 
from SARS and MERS in the order of gastrointestinal symptoms. Our results 
support the notion that fever should be used to screen for entry into facilities 
as regions begin to reopen after the outbreak of Spring 2020. Additionally, our 
findings suggest that good clinical practice should involve recording the order 
of symptom occurrence in COVID-19 and other diseases. If such a systemic 
clinical practice had been standard since ancient diseases, perhaps the 
transition from local outbreak to pandemic could have been avoided.