17. This is a figure composed of 72 identical small quadrilaterals, some of which have been infected and turned black. When a healthy small quadrilateral (white) has at least two adjacent small quadrilaterals infected, it will also be infected and turn black, spreading sequentially. Therefore, at least \qquad additional virus sources (i.e., black small quadrilaterals) are needed to infect the entire large figure. (Adjacent means two small quadrilaterals share a common edge).
【Analysis】As shown in the upper right figure: The four areas marked with red shading have common characteristics: 
For example (1) the red diamond where the red shading is located, even if the outside of the red diamond is completely infected, the inside will not be infected. When (1) becomes a source of infection, the same logic applies to the other three red shaded areas. Therefore, at least 4 additional sources of infection are needed. Of course, after calculation, with these four, the entire graph can be infected.