What test if one group is only yes

General
1

I have the following data set:
Patient Disease1 Disease2
1 Yes Yes
2 No Yes
3 Yes Yes
...
270 No Yes
271 Yes Yes

Obs: all patients of disease 2 have yes
What kind of test I can do to verify if disease 2 is related to disease 1 and statistically significant whe I have only "Yes" for disease 2 ?

2

f(x) = y

x the data frame of observations of Disease1 and Disease2

y the probability of observing Disease2

f the function that predicts Disease2 based on Disease

In statistical notation P(Disease2|Disease1), the probability of Disease2 given Disease1 is the same as P(Disease1=Yes) and P(Disease1=No) for all occurrences of Disease2 in x. Nothing about Disease1 makes Disease2 either more or less probable, given the assumption that the distribution of Disease2 in the population is the same as the distribution of Disease2 in the observations.