Solution

The number of ways one can choose 3 adjacent houses from 12 houses in row is 10. The choices could be

$$$$\begin{matrix}\{(1,2,3),& (2,3, 4),& (3, 4,5),& (4, 5, 6),& (5,6,7),\\ ( 6, 7, 8), &(7,8,9),& (8,9, 10),& (9, 10, 11), &(10,11,12)\} \end{matrix}

The total number of ways in which 3 houses can be chosen from a row of 12 houses is $C_3^{12}$

When each choice is equally likely, probability of the three houses being adjacent is given by $\dfrac{10}{C_{3}^{12}}= \dfrac{1}{22}.$

There is chance 

(221​) that even though virus is not contagious, three adjacent house have virus. Thus, we can't be sure that virus is contagious.

Answer

(221​); No

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